From my training -- many bangers are going into the infantry, MPs, or a similar line MOS. Others go into support positions, like mechanics. In short -- they're going into the normal MOSes based on their abilities and the needs of the services. Not surprisingly, few of them have the combination of characteristics to move into any special warfare or elite unit. (After all, that's why they're called elite units!) That does leave some that do -- just like some bangers go to college, but they're the minority. And, I suspect, you'll find that more of that subset are the ones who are using the military to get out of the gangster life. (Yes, there are people who manage to use the military to turn themselves around; I'm not at all saying that none do.)
But it's not the high-speed stuff that's the concern. It's not even the training in and of itself; as you said, there's plenty of places to learn much of that outside of the military. The real concern, and what's kind of new about it, is that these guys are going into Iraq or Afghanistan or some of the other places we've got troops in harm's way, and they're being blooded in fire. They're making that trained to experienced transition; they're using the tactics under pressure -- and bringing that experience back to the streets of the USA.