think that the idea is that your "standard" feed will die out, as people will post in specific communities for specific subject. and post in communities don't show on your Blog. so if there is for example appics community you can post 5 photos a day, and non will be on your blog, it will only be on it's community. So Cross post is an option for you to post in some other community because you think it fits there as well. You made a great post about Cats in Animal lovers community, and i think that it would be great for Cat photo community so i Cross post it.
Changes are hard, i used beta for some time, and i still first go to "regular" feed, but when communities filter a bit, and get more people, people will post in them. also autovotes will have to change. and if we ever get MST-is it will too change communities.