An attack that does "damage" is very abstract; it could be any number of things. A trip that puts an opponent on the ground is arguably much less abstract, analogous to a sword slash that cripples a leg. Sure, there's more than one way to do that, but there are way more openings to just "do damage" to an opponent.
and the problem with that abstractness is the GM must keep coming up with weeny damage descriptions to justify why your longsword hit didn't kill the guy, as you hit him for the 5th time.
I think the problem is rooted in the OP's comment. If I recall, he said he doesn't know much about martial arts. Wheras most of us are fantasy/medieval fans, and have seen enough sword play in movies, to envision how sword combat works.
I suspect folks who don't like Trip for it's suspension of disbelief are simply lacking in experience with soft style martial arts. The Segal movie reference was apt. They live by redirecting the kinetic energy of your attack and using it to throw you on the ground, trip you, disarm you, etc.
In fact, the best way to beat an aikido guy is to not throw any attack at him.
So what I'm saying is, if you don't like trip because it doesn't seem realistic, then you are simply inexperienced with the real fighting style that would translate to it.
Now if you don't like trip because it's overpowering in your game, that's a whole 'nother problem. Given some folks reference to the TripMonkey, I suspect they're latching onto this thread because it supports their "tripping is bad" mentality. What I'm talking about isn't about the game rules, so much as how tripping can be a very realistic fighting style.
PS. I can beat a junior aikido student, because I have more fighting experience, and all I have to be is faster and feintier than they can handle. I can not beat a senior aikido student, because the style is VERY effective at thwarting attack.