The LJ client I use allows you to write as much as you want whenever you want to - it will only actually be able to post when you get online. The advantage I've seen is that I can write out a really long story / rant offline, then plug in just long enough to press the little "post" button. Otherwise, it's not much different.

Oh, but it does save you that extra step of going to LJ.com and *gasp* logging in. Dunno if that's a deal breaker for you or not :D