The reason I'm okay with things like trip being encounter powers is that I thought 3e's philosophy regarding trip, disarm, and similar maneuvers made sense, but didn't work out very well in actual play.
Trip was balanced around the idea that it conferred an advantage (denying your enemy a full attack by forcing him to spend a move standing back up) over a normal attack and thus should be hard to do with specializing in it. It was so hard to do reliably without feats that people generally didn't bother with it, and with feats, it was effective and reliable enough that people spammed it if they had the specialization.
Neither of these is desirable from a gameplay standpoint. Making trip difficult to do and risky means that, if anything, it's -less- likely to be attempted if the situation is getting dire, because the risk of failure and being tripped in return will make a bad situation worse. And if you're good enough at it to do it reliably, you'll use it every round, more or less. And why shouldn't you?
From a "cool, exciting fantasy combat" perspective, attacks that knock people prone are supposed to happen more often than never, but they're also not supposed to happen continuously. How many protagonists in fantasy literature just spam the same move over and over? It makes for optimal gaming but poor storytelling and boring entertainment.
Trip as an encounter power will probably be quite reliable. You just can't use it every round. The game is, in fact, limiting your options in that way, because it's the best way to make trip simple, reliable, and effective without at the same time making it so good that fighters spend the majority of every combat kicking people in the shins to knock them down. It's a limitation in options with the intent of making DnD combat more like "cool, exciting fantasy combat" from books and movies, and less like a video game where you spam the move you've built your character around being the best at.