Edit: Blah, Hoof beat me to it....
Well, I still made another point so I won't delete this post.
@Choli: No, the person who sends can't be flooded as he's the one who's sending the files.
The other person, however, can be flooded with file transfer requests, and if Plus! receives them all instantly, it could accept hundreds of requests per minute. However, MSN Messenger can't accept more than 5 simultaneous transfers or so, so I don't see a big problem there...