On occasion, a deal needs to be vetoed to protect the stupid. There are owners out there that are weasels and will try and go after the rookies to FF. The rookies and the stupid need to be aided on accoasion. You can't simply say... well, too bad newb, learn as you go and hand another guy a championship that is unearned.
Those are the only 2 reasons I ever have to veto a deal.
If there is a stupid owner in your league then every single owner has just as much of an opportunity to take advantage of them as everyone else.
Its your job to be the first owner to get him to do a bad trade before another owner does it.
There are plenty of stupid owners out there who aren't FF rookies. Some people have played for years, but they just don't "get it."
In my local league that I play in every year with the same guys there is one owner who always makes bad trades. He's been playing for years and knows what he is doing, but he just doesn't "get it" and does bad trades.
I recently got him to trade me Ahman Green and every other owner in our league said, "I raped him." But no one veto'ed the trade because it wasn't cheating. It was him just being a moron.