If you don't limit trips/disarms/sunders/wounds/etc to crits, what's the limiting factor? On paper it sounds cool for your character to have an attack that attempts to stun the opponent. In practice, it slows the game down to roll that check/save every single time you attack, it's lame when it fails and your character does nothing but it ruins balance if your character can do it often, and it's also kind of lame to visualize: 'oh that's Trippington Von Stunlock the fighter, he just goes around tripping and stunning everything we meet all the time, that's his thing'.
I find that critical hits are a great limiting factor. In 'reality' if it were possible to just trip a guy you were fighting anytime you wanted to, that's all you would do; but it's not possible--most of the time you just do normal punches and kicks; you have to wait for your chance to take down your opponent, set it up a little, and accurately execute the right technique at just the right time--iow a critical hit.
At the same time I don't mind too much about the swinginess of damage. Fights are dangerous, swingy affairs. Sometimes you get an 8 second KO; sometimes you go 15 or 25 minutes. You never know what's gonna happen in a fight; that's what makes it so exciting.