Which one? I have always used end plate adapters, like the 2nd item down in the list, but I'm wanting to learn a little more about the rail mounted sling mounts, like the 3rd item down on the list on the magpul site.

I think the balance and handling would be fine, and I think it'd help raise my longer barreled AR's a little higher, but I'd be concerned about it damaging my rail by putting that much stress on it. No, it ain't going to "break", but I'd be concerned about it marring or peening the rail.