Hang on, hang on, lets get some bits straight here......
Yes, a bench polisher is better as its more stable and you might get a guard with it ( your going to want to rip off after 20 seconds guaranteed)
As for speed and the previous comment about power....
clarke 8" polisher is £117, 370w and spins at 2850 rpm
Cheapo clarke corded drill ( exactly what ive been using at home for last 15 years).. is now £15, 550w and spins at a constant ( you guessed it) 2800 rpm.
Even the clarke 6" bench polisher with a better motor is only 500w, and thats £150!
So i would say a drill in a workmate is fine, PROVIDED, its in there well and it has a trigger lock.
To be honest all rotational polishers, grinders etc all carry a degree of risk due to the open nature of the wheel....its why there is a whole special section on use in engineering h&s....its boring, but when your using a 3 phase 12" polisher, that will pull your fingers off and throw stuff the length of a silo.....worth knowing.
Just keep wearing the specs and expect a grey face!
Be safe out there.