Ribs,
You're absolutely right, however, I believe Monkey's question relates to us being able to put our opponent on AAxx. In your scenario, where our opponent isn't always playing AAxx, two pair can and will certainly cost you. NO Doubt. However, here is a hand I played several days ago that I think relates pretty close to the topic at hand.
http://www.pokerhand.org/?1381026
Opponents RR PF from the blinds screams one thing at this level. I elected to just call his rr b/c if I miss the flop I can always get away with an extra couple bucks. And even though I know he's flopped trips it was the best ace that could have hit b/c I can be like 95% sure my flush draw is live. Plus I'm playing very shortstack so my options are few and far between.