[Kevin Ray]

What's to stop an alliance from just (beyond the regular field set-up time limit) switching the bots in the last few seconds to split the difference and at least get two cans. Or in a similar scenario the faster alliance switches their bots to ensure they get all four cans.

The rules account for this. The higher seeded team must place their robot first. Yes, this is counter to what you would expect to be the advantage of being the higher ranked team.