It sounds good on PAPER, but - Chances are you play a deck with some element of offensive inside. Not just to keep the enemy down while you build your tower or resources - then you couldn't afford your hand replaced with cards irrelevant to your victory type. You aim for the enemy tower, hands down.
When you need rares to win by tower destruction, the opponent plays a Tower Building deck(or Adamantine citadel deck, but Ac is more of a theme than a type).
While the card effect would be balanced, without question, would you want to give your opponent +25 tower for free? Some attack cards give the opponent wall since you can have a reasonable enough attack output in your deck to gamble on their raised castle, to make the benefit to the opponent minimal to none.
The same doesn't work with Tower. Tower not only is worth much more than wall, your entire objective with a hand full of attack cards is to destroy the opponent's tower.
Only suicidal yolo'ers would play your last proposal. A comparison could be a Brigand card who gives the opponent resources. The card itself works against their own main purpose.
The card would be niche to the point of total obscurity.
hit enemy tower instead of burning all those bonus recruits just to negate wall gain.
Again, correct at the face of it, but consider.
You have the opponent the Tower. You didn't just give them delay in the form of Wall. You might as well have won them the game. It makes no sense for you to do that. We'd go from a few people playing it to essentially no one playing it.