If you've read this blog for any length of time, I don't think it's a secret that at one point I wanted to be a writer of fiction. (Technically, I suppose I am still a writer of fiction, but I'm on a looooong vacation. My skills are getting rusty.)
But one of the things I love writing is the mash-up. You have a genre from column A and a genre from column B and you mix them. In a loose (okay, very loose) sense, it's how Shakespeare created Hamlet, or Romeo and Juliet. The former is clearly modeled on the revenge morality plays of the time while the latter is a mix of comedy and tragedy, though it ultimately settles on tragedy.
Currently the vogue is for RPG systems that model the genre accurately, and to a certain extent I agree with that. If a particular element or aspect give you the frisson of excitement, you want to provide exactly that element in your gaming. Makes sense to me.
What if what thrills you is, oh, adding guns to your bronze-age fantasy? D&D resisted adding guns for the longest time, and even now it doesn't handle firearms particularly well (though with D20 Modern they did take a kick at that particular can). Or having mystery elements in your superhero game? Or doing a story about changes in personality set during a zombie apocalypse?
Sure, you can do them, but the system doesn't particularly model them well.
Granted, a generic RPG, like the Hero System or GURPS, might not do it well either, but it's guaranteed to do it consistently.
It's a balancing act. If you're introducing just one thing the current system doesn't do particularly well, then you might as well house-rule it. ("Uh....when your superhero sees Lovecraftian monsters, he has to roll against Will.") But if you're bringing in a bunch of things, or if you see the possibility of bringing in a lot of things, then a generic system might be what you want.
(And by "generic system" I'm even including something like GUMSHOE, which is a generic basis for systems. You can easily incorporate the NBA chase rules into your Trail of Cthulhu game, for instance.)
I know over on the Writing Excuses podcast they're busy talking about combining what they call elemental genres. This is something similar: when you know what kind of response you want from the players, what kind of flavour you're going for, then you know whether you have a particular genre but heavily modified, or if you have a new thing, in which case you might as well go for a generic system because you don't know what kind of response you're looking for.
Wait, that sounds kind of uncertain.
Except that sometimes roleplaying is kind of uncertain. The GM brings something to the players, and the combination of players and GM makes something new. Neither side has control over the result; either side can choose to scuttle it. It's something you do together. You might have a great history for your world, but if the characters never see it, it lays there. An NPC might have a tremendous fear of wights, but if no wights show up, big deal.
So a generic system is really useful if you're not quite certain what you're building together. Maybe after you've been at it for a while, you'll find some other system that makes it work better, that emphasizes the parts you really like. And that's okay: it's okay to switch game mechanics halfway through. (I used to joke about it, when we were switching from a game system where the attack had to be the last action in your turn to one where the order didn't matter: "He hits and runs away, because we've changed game systems.")
So there is a place for the generic system, even if you're heavily into specialized systems. They're the block of marble you start with, carving away everything that doesn't look like an elephant.