Dear Miss Snark,
When an agent asks for an exclusive on a full manuscript, do they believe they have the right to expect a "yes" if they offer representation to the author? Might they withdraw their offer if the author asks them to wait a month or two for a decision (because he's since had requests from other - preferred - agents, and wants to send them the full now the exclusive has expired, and wants to see if they make an offer)?
If you think you can shop around and do better, have at it. I might be here when you get back, but I probably won't. If I know my colleauges are saying no to you, I'm MUCH less likely to think I should say yes. And if you aren't all that eager to work with me, well, ok, there are 100 people lined up this week alone who think I'll be just dandy.
I expect people who have sent me full manuscripts are willing to sign with me. Absent any startling revelations ("oh my dog, he's really Miss Snark" or "oh dear dog, you aren't going to call me every day and review every detail of what you've done for me?") you should know you're willing to sign BEFORE I invest time reading your work.
Agents love auctions and bidding wars but only for projects they represent.