In fact, that's the only thing it does -- there are no options, for instance, for Resto Shammies or Holy Priests (although Dan is working on healing gear rankings -- Holy Paladin is one of the newer options). All the site does is look at the stats on gear (and your own stats, if you punch them in), and then rank all the gear in the game to tell you how much DPS that piece will let you turn out. And if that's your goal, the thing can be pretty helpful -- I saw a few quest pieces on my Hunter that I could pretty easily upgrade to if I just wanted to up my DPS by about 10 or so.
The only problem, of course, is that it's DPS only -- if you want to put together a set that is all about plus healing, or max out Intellect for whatever reason, you're still going to have to parse the gear that shows up in the ranks (which is one reason I liked Gear Wishlist so much -- you can rank for MaxDPS, or switch the rank over to item level or the Wowhead ratings for each piece). But once you've figured out what you want your character's main goal to be with gear at endgame, all of these sites will do their best to provide suggestions for upgrades to you, from all the stuff available in quests and PvP at level 70, up to the highest pieces you can find in endgame raiding.