From what I remember of a previous similar discussion, there was a lot of resistance to making it easy to cross-post to other communities on the same site, because then you just get identical posts spammed, as you say. Communities usually have slightly different purposes and standard formats, so it's good to encourage people to tailor their entries, though obviously people can work round that.

However, being able to crosspost to a mirror community on LJ would be very useful. (For examples of people doing this manually, see metafandom or sg_five_things.)