There are two reasons to veto in my book.
1. Experienced player bends over a Noob. No noobs in the leagues I'm in, which leaves...
2. Owner acquires cash in a trade, and thus has a cap advantage over me.
I'm pretty hardcore, so most "I'm throwing in a million to cover the salary" deals go through with probably me being the only one to veto.
Every now and then, veto. Usually, large amount of cash, and someone was to dumb or lazy to budget correct.
Because, in the end, most times cash fixes the problems of the dumb and/or lazy.
"I don't have enough to sign _________ IFA/first round pick"
"I don't have enough payroll to sign an FA/bring up someone from AAA/make this trade" (which I usually point out that there's coach or prospect budget to transfer, but I digress)
Dumb, lazy, or smart enough to know that I don't need X+5million in salary during budgetting because some sucker is going to give it to me.