I've lost count of how many comms I'm on and how many crossposts I see on my flist, but I really don't give two hoots. If I've seen it before I just scroll on past. I regularly get back to 180 posts every morning when I read my flist - it was 240 this morning.
Why do people hate crossposting so much? I'm really interested to know, because it doesn't bother me in the slightest.
What I think would be a really useful tool is a prog that could compare comms for you and tell you how many unique memebers each had. I tend to look at membership numbers or speciality when crossposting because if I crossposted to all the comms I'm a member of it would take me all week, and it would be really useful to know if any of the comms are redundant for some fics :).