Well, I can sort of understand the idea behind this. Twitter obviously want to broaden themselves, but the question is whether it will really benefit them enough to justify this acquisition.
Posterous is apparently going to remain as is, so there won’t be an obvious connection.
The fact is that Twitter will not benefit nearly as much as Posterous from this deal. Although Twitter will gain a lot of expertise, Posterous will be the one to gain exposure and as a result a lot more users.