I didn't have time to comment on this when I first read it.
I feel this is a problem with games like 3E that tries to generalize maneuvers. Once you know how to trip, you can always trip-there is no need to wait for an opening, and the opponent has no way to look out for and guard against your recurring trip. Which can quickly get boring and repetitive. I'm going to list some possible ways to combat this situation:
Feng Shui had a "boring and repetitive" rule. If the GM judges and action repetitive, he can assign an arbitrary penalty. Feng Shui is supposed to be cinematic; your move was not cinematic enough and got cut. I'd append that the player should be given the option to try something else instead with his action, but that's me. This rules is simple and works in any game but not with any group.
4E tried to combat this with dailies and encounter powers; you can trip, but enemies will only fall for this once per fight. I could bye this explanation, but many (including most of my players) could not - it lacks verisimilitude.
There was a little-known combat card game called Highlander. A card game can solve this in that you have a random hand of possible maneuvers. If you read these as openings in your opponent's defenses, it is a solution to the problem. You can only trip when you have a trip card in your hand. (Highlander did not have a trip IIR, but you get the drift).
Fighting Fantasy was a book dueling game - each combatant was represented by a book with a position/stance on each page and a maneuver card with color-coded maneuvers. It was like advanced rock-paper-scissors. Certain positions were restricted; "Do only green or yellow next turn". Certain of these restrictions made you vulnerable to certain attacks - like trip. You could try to set the opponent up this way. Also, if one player consistently did trip attacks, the other would get wise to the tactic and use moves that were not vulnerable to trips. While interesting, this example is way too complex for rpgs in which you fight a lot, especially open melee (as opposed to duels).
This is where the die roll comes in - it shows whether there really was an opening or not. As you'r skill, strength or agility does not vary over time, the die roll is there to simulate all the factors you do not have control over - is the arms, legs, etc in the right place, is the target off-balance or not, at that particular moment you look for an opening. A failed roll might as well represent that there was no opening to exploit.
In Computer Science, there is a concept called "Lazy Evaluation" (http://en.wikipedia.org/wiki/Lazy_evaluation), which basically means "dont calculate it unless you need it" - for example, when you simulate a scene using computer graphics, you can get a pretty good approximation by just calculating the path of the photons that enventually will hit the eyes of the imagined beholder's retina - and approximation called raytracing, as opposed to radiosity - but I digress..
In the same matter you can be lazy with the rolls - you don't need to roll for an opening unless someone is looking for an opportunity to trip/grapple/disarm. But I think this can cause a mental disconnect - you can easilly see it as that there *are* no opportunities unless you roll. When I played Shadowrun, there was a fierce table discussion about the fact that in that system you could roll for dodging when someone shot a firearm at you. Took a long time for people to accept the fact that you dodged all the time and you only rolled for it when it mattered. It really helped when finally Lazy Evaluation was brought up - since most of the players had a CS background, that argument finally got acceptance.
The thing about using the die roll to simulate the presence of openings and opportunities, is that as you improve in an ability, such as "trip", more and more narrow openings become exploitable, and you get more and more opportunities - as seen in a larger and larger part of the possible die roll results becoming sucesses. Not only are you able to use more narrow openings, but you can create openings when needed...
Which brings me to one of the problems I think AEDU has with getting accepted for maneuvers...
Why do we collect XP and level up in this game?
After all, the XP progression is just a death clock for a (hopefully) beloved character. At a certain XP sum, a certain level, that characters life is over, the campaign ends, and new characters are rolled up.
Well, the big draw is supposed to be that the character is supposed to become better as you level up. That is one of the big expectations people have of a level-up system.
As in for example, if you see yourself as the great maneuver expert, you do feel better when you get more and more opportunities to use your ability. With AEDU you dont gett any more chances than the number of maneuver ED cards you have, but with a die roll, if you can keep it improving ahead of the curve, you feel you get more and more opportunities.
I think that is why people simply want more chances than AEDU limits them to to be able to feel like they are becoming an Expert at something, whether it is trip, or disarm, or throwing big honking blocks of stone...