Absolutely. We should blame inadequate teaching, not just punching the air.
I don't know if a tight fist 'strengthens' the wrist, but it certainly contributes to keeping the wrist straight. Try it, folks. Clench your fist loosely and watch how the wrist wobbles. Not what you want on impact. Now try my variation, known as the 'Kisaka fist' after the JKA honcho who empahsized it: clench the fingers tightly and instead of a normal thumb wrap turn the thumb sideways a little more so that it exerts more lateral pressure upon the entire chain of fingers down the line. You'll notice your fist really compacting nicely if you're doing it right. Now, the real trick is to be able to quickly assume this fist quality upon the moment of kime, but until then keep everything in the delivery with shoulder and arm nice and relaxed. It's not an easy thing to do, but it's necessary to learn. If there's tension in the arms throughout the punch, you're doing it wrong, and have what the old-timers call a 'dead' fist.
Heh Home theory vs. practical experience here, I'd say

. You go, girl.
Amen. And so on.