You've probably heard this question before. But as a writer eager to upgrade to the title of "bona fide novelist" -while I still have some of my youth- I need to know what's the worst that could happen if I ignore an agent's request to exclusively submit my entire manuscript to them?
Well, when they find out, they throw your manuscript away without reading it.
Or maybe they never find out and they make an offer to you and become your agent, and you're signing your first deal at the bar, and you mention it, laughingly, and your agent's face slackens a bit when she finds out you think that was ok.
I am not a big fan of exclusives. I don't ask for them. I don't think you should send stuff on an exclusive basis for longer than two weeks TOPS on a partial, or a month TOPS on a novel.
That said, if you agree to send submit your work to an agent who asks for exclusivity, and you don't honor that, it says something about the kind of person you are. Is it the kind of thing you want said?