It's growth uses up nutrients that other algae might otherwise use and other algae are less likely to attach to coralline algae then they are to bare rock.
As far as looking nice, thats a matter of opinion :wink:

when it gets so thick it can no longer hold itself up, it looks pretty tacky

Sorry for the crappy photos I was fighting reflections but this is what I mean, these is pics of the back glass and one of my overflows

http://img.photobucket.com/albums/v2...ralline001.jpg

http://img.photobucket.com/albums/v2...ralline002.jpg

Ok well I guess thats good for me then, so far I like the look of it, but I think Im going to scrape if off the glass. I dont really like the spotted look.