Uh, the guys at valve playtested competitive portal games, and mostly concluded it just wasn't fun.
Apparently, a big issue they had was people getting incredibly disoriented when they fall into a portal, which is bad enough when you yourself put it there, but seriously confusing when it's unexpected.
And, it just seemed not to be much fun for those trying to use a portal as an offensive weapon either.
I don't know. I guess it might work.
But I'd be really surprised if valve hadn't tried it at all.