In item #6 this article quotes Ryan Sarver, Director of Platform at Twitter, explaining why Twitter doesn't extract snippets from feeds: "[PubSubHubbub] is fast enough, RSS is not. PSHB isn't broad enough yet."
Nevermind the speed admission (which is in itself a small victory)-- What does he mean by "not broad enough yet?" PubSubHubbub is supported by every major blogging platform; used by location services; powers cross-site follow on microblogging sites; provides publish and subscribe for social networks; enables a range of search and track features; produces real-time feed analytics; has clients for popular languages and CMSes; has existing Twitter-bridging tools; and there's even a prototype of what Twitter's support for PubSubHubbub would look like. (I'm leaving out a lot of links.)
I think I understand Ryan, actually: He's in the marketing department and just doing his job. Since Twitter aspires to be the infrastructure-- the "nervous system" of the web-- they need to provide a compelling reason to integrate directly with Twitter (lock-in) and not with the open web. This is the platform game (of antiquity). The easiest way to convince others to put their eggs in one basket is to spread FUD (fear, uncertainty, doubt) about the other platform that's out there.
This FUD is so transparent it's kind of refreshing.