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1. Do you have enough space to keep that much junk lying around? (Calling a robot junk is questionable but often applicable) my school has about enough space for a backlog of 2 robots so thats what we keep. really thats only the result of lazyness. if we werent lazy there would be no robots out in that shed.
2. What are you going to gain? if you want to train drivers, only keep a drive train. if you want to train programmers, again keep a drive train. if you only have a drive train this is much easier to store.If you want to use it to demonstrate to sponsors scrap the old robot and use the new one. Sponsors like to see the most recent robot.
3. Parts. old robots have so many valuable parts in them that you can reuse. my team has used the same banebots transmissions for 3 years running. old robots house a plethora of useful stuff (including the CRIO and other electronics). Really keeping old robots is a matter of are you willing to keep buying a new crio every year or retrofitting the old control system to the robot.

Basically if you have lots of sponsor money and sponsor space then keep the old robot but if you just a team running out of a school with limited space and budget then definetly RECYCLE(cannibalizing sounds so savage) the robot.