2$ a month is much less than 15$ a month - especially WHEN ITS OPTIONAL
I'd be willing to say that powergamers will easily be able to spend more than $10 a month, probably closer to $20+. So how you define optional is up to you, but I know that econmic reality dictates things a little different than you might imagine.
So if you're a casual MMORPG player then Guild Wars is for you! Otherwise if you're one of those people who devours content and whatnot, other MMORPG's might have more bang for the buck.