I'd even go so far as to say that other genres DON'T keep their online play interesting. Using your logic, I could say that, since people still play Everquest, it's managed to keep its online play interesting as well. Only, I'm fairly sure it hasn't. People will keep playing online games whether they stay interesting or not, and that includes both MMOs and online FPS.
Hm, well; that's your opinion on the matter.
I was never a big CT player but...
...As a TFC (and TF2) player myself, I find myself playing through the same content over and over. There's no pressure, no grinding, no gear requirements, no subscription fee; just log on and have fun.
With a multitude of classes to play, and map modes; it's hard for things to get old; but now I'm going off topic.
This can't be applied to an online action RPG type game? Include all the siege gameplay, all the pvp battles, and all of the PVE content and remove the NEED for it.
An advantage and marginally stat increase is a really lousy reason to get people to play these things, they should do it because they want to, like playing the next gen version of Golden Axe, only online with some friends. Pretty bad example; but I think you get what I mean.
Put a fee on it, and you can even offer purely cosmetic upgrades to people who do well... but maybe the masses aren't ready for it. Maybe It's just my own opinion here.
Just like you think that online shooters, or any other online game does a poor job of keeping people playing for whatever amount of time.