The issue with almost any XBLA game that's multiplayer focused is that it's destined to die. Exception being Duels of the Planeswalkers which is always rather active.
Tons of great XBLA FPS games have gone down the drain, most notable being Cell Factor: The PK Wars. It was really fun and almost felt like a throwback to the older days of FPS gaming but it was, of course, dead. I'm sure Perfect Dark has no real online community, same with Blacklight: Tango Down probably too.
Odds are for $15 I'd play it a few times only to have it end up a dead zone and it a waste of cash and space on my hard drive. And for a game that's really dependent on 6v6 team based shooting, it certainly won't work well with small amounts of people.