I haven't really ever seen a sub give $15/month in value. Sure, at one time I thought it did, and with the sub model being the only game in town, what were you to do? Now, after games like GW 2 showed that B2P works just as well content wise, and F2P games have a ton of content as well, why waste your money? In a perfect world, a niche game that perfectly supplied what I'm looking for would need my $15 monthly to keep all the great content going every month but that's not realistic. Every game today has some form of collection garbage as the ongoing form of content to supply the happy masses until another expansion is made. I don't feel like paying $15 monthly to do my 'chores'. To get my copy of the 15 slot bag or the new shiny purple, etc. that everyone else is getting.
EQ 2 had those adventure packs and I thought it was a solid idea at the time. If every 3-4 months, I could get 3-4 new zones of well made content for $10, I'd sign up every time...with an original B2P model.