we usually have a line of guys at the top of the first hill on one of our national marathon series, all clutching broken SRAM rear mechs.
One of them has even been known to leave a spare bike up there, and i've seen several of them handed rear mechs........... never noticed any shimano mechs, even the years i've spectated (good place to watch, they go past virtually the same point 5 times in the space of the first hour, then another 3 or 4 times during the remainder of the event)