Well, I do mostly play on pubs, yes.
But even so, after a spy picks up the intel, he's got to fight his way through the entire base, either going through the straight staircase and running into the most popular sentry spot in the level, or through the spiral, fighting off every member of the opposite team, who'll know exactly where he is. And every enemy he kills is going to respawn instantly and immediately come at him again. I don't care how good a spy you are, that's a pretty tall task.