Okay, first, although I did say the class trait provides access to the basic moves for that class, I was really still thinking of the older version of Dungeon World in which each class has more or less a primary move: the fighter can Bend Bars/Lift Gates, the ranger can Hunt and Track, the wizard can Cast a Spell… Obviously even in that version it wasn't quite that clear cut, for example you'd probably want a cleric hireling both for casting cleric spells and for turning undead. But that's where I was starting from, and I think for hirelings that's something you'd want to spell out, that a hireling has a specialist skill that you have some way of accessing, though not necessarily automatically and without cost.
Because I'll tell you, as a player, the reason I want hirelings is to help me handle unusual things that otherwise would just block or kill me. I think I mentioned that in Jason's AP thread but not in my hireling post here (or on my LiveJournal). There'd be a situation like, say, my wizard's been hit with a chaos curse, and to remove it, I've been given a quest that involves hunting down a peryton. Well, that's a trek into the wild to hunt a monstrous animal; I want to get the aid of a ranger. Or there's a known threat of undead skeleton warriors plaguing a trade route; I'm not going off to deal with that without a cleric in tow. [And since after this point I ramble on about a related topic, I'll say here that I do like what you're thinking, Sage, on how to incorporate this.]
What I'm not interested in is having nameless meat shields. I'm a hero! If my party doesn't dare venture forth without expendable pseudo-people, well, we don't deserve the title heroes. But more to the point, consider this. One of the three agendas in Apocalypse World is "Make (it) seem real", and one of the principles is "Name everyone, make everyone human." Dungeon World's agendas and principles are somewhat different and don't include those two specific ones, and that's fine; for the old-school dungeon-crawl feel, you don't necessarily want to make everyone human. But, if you make the hirelings real people, with names and cares and interests, then you open up lots of interesting possibilities. At the most basic level, hirelings become opportunities for the DM to use the moves "Use up their resources", "Put someone in a spot", or even "Show a downside to their class, race, or equipment." But also, when hirelings have names and personalities—even if they don't, in fact, offer special benefits of their class—the players will be more interested and more involved in the game, maybe not all the time, but at least some of the time.
In the first and ongoing Apocalypse D&D game that Tony's running for me and his brother-in-law Gabe, we started out with two men-at-arms hirelings and have picked up both a cleric and a paladin along the way, and they have provided a lot of interesting drama and hard choices for us as players, simply because Tony's played them as named people. I decided early on that my fighter was the fifth son of the local baron, and so the men-at-arms were not just hirelings but my father's soldiers. Suddenly that made me responsible for their well-being, which Tony has gleefully exploited. We found a section of the dungeon that's a lost and corrupted dwarven temple, and our cleric happens to be a dwarf who declares it's his duty to cleanse the temple; do we part ways with him, or aid him? If we'd simply parted ways, would he have been willing to help us out later with healing needs if we came crawling back?
That's where I'm coming from.