I'd really like to see this functionality (automatically cross post MT to Live Journal) built into MT:

http://www.kodekoan.com/project/MTLJPost

I've used that for a couple years and it's a great tool, but it's pretty much EOL, they developer no longer plans to do ongoing support. It's a pretty simple plugin and seems liek it'd be SUPER easy to hook these together now that SA owns LJ.

Why should this be built in? It's perfectly appropriate as a plugin. (Cross-)Posting to Livejournal is not a function of a content publishing system. If you want 6A to produce an "official" LJ cross-post plugin, that's another matter, and much more likely. There's already a Vox cross-poster being worked on.
You have to consider that if LJ dies, then MT has some useless function built into it until the next update. Or, what happens if they change the way that LJ accepts information in the meantime? A plugin is the best solution in pretty much any situation here.

On the other hand, what's the problem with this existing plugin? As far as I can tell, it was updated not two weeks ago.