The most obvious thing is that you can leave him on the mat indefinitely with Reserve.
Once you have learned all Teacher's tricks you really have minimal use for having him in your deck and he becomes just another dead, high value card (e.g. like say Pooka or Altar when they are no longer useful trashers). A duration version would be inferior because you would have to draw it every turn.
And you often do not want to waste time getting all 4 tokens out. Champion makes +action pretty useless outside of edge cases like Snowy village or Diadem. +Buys are normally not worth a lot with Kc/Squire in the mix or something like Festival/Margrave. Even +card and +coin can be worthless (e.g. Festival/Lib and mass Hwy's respectively) rarely. Far more often you are getting close to game end and the odds of Teacher being the last stop card in your hand are not worth the highly marginal utility of one or more tokens. Sometimes, you can build a whole deck off just a couple of cards (e.g. Minion/Peasant/Soldier/trashing) and have no logical place to stick the other tokens.
Relatedly, one of the nice things about callable reserves is that you can bank them for later for cards that care. Most obviously this happens with Hop. Say I setup some sort of engine, leaving the Teacher on the mat gives me an 8th distinct card that I can call back only when needed to spike my "in-play" numbers. A few other, far less useful, cases are out there (e.g. maybe getting the count right for a Leprechaun or retriggering a Magic lamp with Treasurer).
Lastly, there is the tactical case. Suppose you 3 Smithies and 3 Margraves in deck with Academy completed. If you draw none of either the first time after playing Teacher, you benefit by waiting another turn to make sure that you put +cards on whichever you draw the subsequent turn. Shuffle dynamics can get hairy, but in general, it makes no difference when in a shuffle you call a Teacher unless it is the turn you will shuffle. In some cases it can be worth it to wait to see what you draw because adding a +card to Village or Walled village does not matter if you have equal numbers of both in your deck and equal numbers left in the piles.
