Cross posting is good when you want to broadcast something, so that all your friends on various networks get the news. For example, a link to your latest blog post or something like that.
But if everything is being cross-posted, then it creates noise.
That’s why while setting cross-posting features, one has to be really careful. You should be selective while cross-posting.
This is the reason, I use Friendfeed to update my Twitter status, Twitter to update my Facebook status, and so on. And I selectively update my FF status.
Posterous allows you to autopost, but in my opinion it creates noise on all the connected services as well as a threat of creating DUPLICATE content. If the content is being autoposted on your blog too. Isn’t it.
But people might have different opinion about it.
Let me know your views about it.