In my mind, only trades made due to collusion should be vetoed. I guess if there's a hugely lopsided trade made by a rookie manager, that could be vetoed, too. However, if there are two experienced GMs who just have different visions, their trade should be respected.
In reality, people veto trades for a number of reasons. I had one of my trades vetoed in a keeper league. I had people tell me that they were vetoing the trade because they had a grudge against me. Others vetoed the deal because it made my team too good. I thought both were chicken excrement moves, but what can you do?
In general, I think that if a rival GM pulls off a great trade, the response shouldn't be to veto. Rather, the GM should try to make a great trade of his own. Blocking great trades for successful teams to preserve competitive balance is essentially affirmative action for bad GMs.