As with many facets of the game's combat mechanics, in-game tactical behavior is nothing like what you'd expect in real life. In real life no general would waste an absurdly expensive defense like a treb against an advancing disposable warrior.

Trebs aren't worthless and useless per se, but they are a grossly expensive and inefficient defense against a skilled attacker who understands the game mechanics. Not to mention that they scream "ATTACK ME" to people trying to rack up huge honor points.

Lots of ATs and a few abatis seem to be the way to go, as far as I can tell.