Helmet or no helmet has been discussed in depth a few times on here but the mention and meaning of MIPS by the OP has interested me.
I choose to not wear a helmet, from my early days in bmx through to the present day, whether ramp riding, street riding, dirt jumping, dh or xc. Thats not to say i have never wore one, if the event or course requires one i will and have wore them, i actually own a few. I would like to say though at the start, that i do not wish to influence anyones decision to wear a helmet or not, that i feel is a purely personal choice and should not be affected by peer pressure etc. I do wish though to point out a few things, some already lightly touched upon here, that are not often disclosed, for obvious reasons, by the companies selling the products.
I learnt a bit about helmets through work, that at the time was climbing and the testing of safety equipment, though the construction, testing and design principles are the same, which led me onto cycle helmets too. As i said above, i have no issue with a person choosing to wear a helmet, of course if someone hit me on the head with a hammer i would feel it more than if i had a helmet on (thought i would get that in before someone else), and i have seen a few people gain massive confidence after putting a helmet on. I believe confidence is one of the most important things while riding a bike, along with other sports and life in general, so if that is what they gain then thats great, i for one would never suggest they dont wear it.
Now i would like to say first that my research was only up to a few years ago, things may have changed but i honestly can not see that, and if history is anything to go by, in fact got worse.
There have been many many trials and hearings over the years for claims and libel where the helmet was, or became, the defining topic. Not one accident/injury has been proved to have been preventable if the user had been wearing a helmet, despite the best testing and experts. Of course the only way to know for sure would be a time machine and to re-live the event with a helmet to compare. So a bit of a mute discussion really.
As far as the helmet itself, you need to first understand that 90+% of companies make helmets first and foremost for profit, a business, and not for your safety. Let me expand on that a little. A company aims to sell helmets, by making them 'fashionable', appealing to the consumer, through looks and comfort, and of course meeting safety regulations for your peace of mind. Now, there are numerous safety ratings a helmet can achieve, and most helmets do just meet them, the lowest one that is. Cycling companies, and other sectors, spend a vast amount of money campaigning to lower safety standards, to aid in their attempt to make the product more appealing. If the helmet met the highest safety standard then it would be really heavy, have no vents and not look appealing in the slightest, not good business sense. Now on to the ratings themselves, the impact force tested on the lowest standard, the one most helmets just scrape over, is the equivalent of a person falling over from a standing stationary position, so dropping the helmet from around 5.5ft. If you add any momentum into that the forces rise considerably and would not meet the standard. Another rating is for penetration, branches, sharp rocks, debris etc. Cycling helmets, along with climbing helmets, can not be entered for testing as they have gaps in the shell, the vents. A common helmet for climbing did overcome this by starting to make plugs for the vents, though to meet the higher standard needed for rope access, which is actually concerned with a persons safety, it had to make a version without vents. It must also be stressed that the above impact rating is only for impacts directly from above, at the time of research no helmet was able to be tested/rated due to design, again, an aesthetic choice by the companies, for side impacts. The only climbing helmet that was is the one already mentioned above, that is not contoured and shaped like all other climbing and cycling helmets, it extends down at the sides and back for protection and therefore can be submitted for side impact testing and certification. Companies spend a lot of money designing helmets to be light, cool, as in well ventilated, and aesthetically pleasing so as to appeal to more people. So... as long as when you crash, you land square on top of your head, not traveling at any speed and dont land on any thing pointy, you should be fine
sorry, back to the issue.
Now, as i said above, what interested me about the OP's post is MIPS. i have seen it mentioned on helmets but have never looked at it, i actually thought it was something to do with the fit. This brings me to another important aspect of helmets. Again, already mentioned by the OP, rotational forces and more importantly injuries. A great deal of research has been carried out regarding the subject, it is agreed that a helmet, any helmet, greatly increases the rotational forces in a crash, obviously with the area around the head being enlarged by the helmet the forces will be greater, and a lot of injuries are found to be from the result of the helmet itself. That is not to say without the helmet you do not get the rotational force, of course you do, it is just amplified by the helmet often resulting in more severe versions of the injury. This aspect i found interesting, the protective item actually resulting in harm not present without. Full face helmets for example, in quite a few instances have caused injuries from the bottom of the helmet striking the collarbone during an accident, obviously not evident without a full face helmet. This applies to all protective equipment, pads etc, and most controversial among others are neck braces, they were banned by Motorcross associations due to the injuries directly caused by them. I have seen races since that a couple of riders have started wearing them again, whether that is through relaxing of the rules or they are medically exempt im not sure.
Off on a tangent, a lack of safety certification does not mean the item is unsafe or does not meet safety requirements, Dainese for example, a highly respected manufacturer of safety equipment in both cycling and motorsports did for a long time not carry any certification on their protective items, purely because the fees for certification are very high/extortionate, so they opted out, which is another point maybe to consider regarding the companies just trying to meet the lowest certification.
Back to rotational forces, peaks, now a lot of companies do make peaks that attach, designed to break away in an accident, to stop further injury, but i see a lot of helmets with fixed visors, built into the design, again for aesthetics etc. Now, the rotational forces created by a visor catching on the ground during an accident are again, as you can imagine, greatly amplified,and again found to be the cause of some horrific injuries. Which brings me to MIPS, i think it is great that companies are trying to design features that help limit injuries, it is definitely an area which needs attention, though it does highlight the issue i was trying to raise, the lack of information/disclosure regarding the hazards and risks of wearing a helmet, or any protective equipment while cycling or climbing etc and, the fact it is a very real issue.
I would like to again make clear, i am not trying to put any one off wearing a helmet or other forms of protection, i do, as previously mentioned, support anyone that decides to, for whatever reason, i merely wish people could make these decisions armed with information regarding all aspects of the said items, and not just with 'a company told me it will protect me', or 'you would have been a lot worse off without one' type reasons.
Sorry for the long post.
Ride long and prosper