Look at it this way. You don't see any large companies handling their
email routing by setting up Outlook Express with a crapload of delivery
rules. You have an entire email 'system' for handling them.
That's not the right analogy! The right one is "does a mailing list server have to be its own SMTP server or can it just connect to an SMTP server the way a mail client would". The answer is that, while many listservs support direct SMTP, it's optional. It does help in some areas like handling bounces, but it isn't a requirement even for large-scale services.
In the IM bot realm, the existence of ActiveBuddy <www.activeBuddy.com> disproves your argument. Since their bots are available on today's monolithic IM systems like AIM and Yahoo, they are obviously not running their own servers since those services don't support multiple servers. Their bots log into AIM or Yahoo just like a client (and that raises the interesting question of why AOL isn't trying to block them. Perhaps they have some kind of special deal.)
I agree with you that there are copious implementation reasons to implement a bot as a transport given the current server; but there are no architectural reasons to (and reasons against it, as I listed earlier.)
�Jens
