dd wrote:But the thing is, this spell isn't a straight polymorph.
Okay, just from this bit here I can tell that you
do not understand the crux of my objection.
Adding features is a lot of work. It's potentially dangerous. Every feature can interact with other features, and each of those has a chance to introduce game-breaking bugs, difficulty-ruining exploits, and spoiler-ridden crippling vulnerabilities. The more features there are in the game, the more combinations of features you have to test. Usually you can silo off things that work together, and ignore things that can't normally come up in actual play, but the proposal you're making would bring the whole gigantic class of monster powers that have never needed much testing because the player can't control them, and it brings them into the player toolkit with all the other player powers. Just using the monsters we have already, we'd need ten million full-time playtesters checking just this one thing for hundreds of thousands of years just to check all the monster powers.
To name just one example, most of the Crypt ending would be absolutely crippled by the player ability to take control of a monster ghoul and feed it some choice equipment. Literally that player could tab the ghoul through the entire end vault full of monsters, and then when the well-equipped ghoul breaks loose just kite the thing until it dies, because ghouls are slow. It would be mind-numbingly boring, but safe.
Or how about an orb of fire in Zot? One-shot most Zot threats, and you're basically immune to anything that can survive that first hit. The body vulnerability is not even hypothetically risky, because you've got a freaking orb of fire guarding your body.
Ultimately, the trouble is that you thought of a cool and flavorful idea, but then made next to no effort in considering the implications of the idea on the game as a whole. The horrendous amount of work you're asking for is probably more complicated than all the design work done on the game up until this point, but apparently anybody who points this out is just a meanie who's trash-talking your awesome idea.