When I bought my last extract kit (Nut Brown Ale) at the LHBS, I noticed the ingredient list (when I got home) listed Irish Moss as an optional ingredient.
Funny thing is, the LHBS guys have to add stuff to the box when you buy it (The yeast, the hops, etc) and they didn't even mention the Irish Moss. You'd think, being that they want to make as much money as possible) that they'd try to push it, but they didn't.
So, yeah. I figure it's not something they consider essential (especially with extract brewing, even though there's hops and specialty grains). I suppose it's a personal opinion, depending how clear you want to get the beer.
I'm planning on a long 3-4 week primary and hoped that most stuff would drop out of suspension on it's own.