Well if it was up to me, i would keep every last one.
But unfortunately my team is a bunch of salivating cannibals that only cares about this year's robot!
If you care about your machines at all, don't ever park them near the shop during build season. Its starts small with things like PWMs, then all of a sudden a bearing is missing, then a wheel..... It will never end.
But seriously, old robots are very valuable as instruments of inspiration, prototyping tools, and rookie training. We still have our 2006 twins intact and they have been everywhere. I would keep at least one around.