One of the pros can correct me if I get this wrong:
Irish Moss is positively charged (ion). This assists in taking some of the smaller proteins out of solution, not to help the yeast drop.
I guess it's addition would depend on the style of beer that you want to acheive.
I was goofing one night brewing(drunk?), and I added some to an ESB recipe. The result was a fine beer, but it was missing some mouthfeel, due to dragging some of the "goods" out of the brew. Definitely not the style I was going for.