Atrios says he's "not anti-convention, anti-convention coverage, or anti-people going to the convention, but there's really no reason for local, national, and international organizations to spend that much money sending people to cover these things."
I suppose it's always worth starting with the assumption that people know what they're doing and asking why they would do something that on its face doesn't seem to make sense. That leads pretty quickly to the conclusion that for the cable networks, intensive convention coverage probably does make sense. In the modern day, the mainstream TV networks (rightly) have decided that there isn't enough interest in the conventions to justify extensive coverage. But there's still much more interest in the Republican convention than there is in a normal random August Monday of news, so it's great for dedicated TV news networks.
For all the writers, I think you have to think of convention coverage as in part an employee benefit. I covered the 2004 Democratic and GOP conventions and the 2008 GOP Convention and had a blast doing it. I don't particularly regret that I'm not going this time around—I'm older, have a different beat, et cetera—but it's still fun and certainly something anyone in the business is going to want the chance to do at least once. And it makes perfect sense for news organizations to invest in their staff.