[seg9585]

I like this idea. During a time in which many engineering companies (especially aerospace) are attempting to modularize their products (to reduce cost, etc) and establish standards, teaching students about modularization would be very beneficial.
I could imagine a rule where mechanism swap-out must take <5 minutes, so the robots would have to be designed to accommodate different mechanisms/payloads with the same robot bus. Wire harnessing, structural attachments, etc would all come into play, and this idea would push pre-match strategy even further.
When mechanisms break, swapping it out with a different type for the next match -- while the pit crew works to rebuild the broken mechanism -- would also make queuing run more smoothly. It gives the pit crew something to do while the robot is in a match, at the very least.