You're missing the context. This is a substantial number of conversations, that are necessarily coming into a *single* IM window. Being able to disambiguate them is not optional, and there aren't a huge number of ways to do that. 1-9 slot numbers are the least-bad solution I have seen to date.

I'm of course not going to bother the user with these slot numbers when they aren't necessary. But the lowest-common-denominator IM integration seems to require them...