*Maximum length for the links so that users don't abuse it.
I think 140 characters is a bit short for English, but I do not think that 280 characters would give us better content in the long run either.
Given that eliminating the limit entirely seems like the wrong answer, I also don't think it makes sense to extend the 140 character limit to something marginally longer; that just seems like pointless churn that would break the expectations of various tools that interact with Twitter and other microblogging services.
Of course, that's just for short text messages. Maybe embedding in hyperlinks, pictures, music, and video doesn't need the same constraints.
[1] http://latimesblogs.latimes.com/technology/2009/05/invented-...
Extending the limit would just make people sloppy.