It's not dropping the knee to the ground, it's bending it anywhere near 90 degrees (most days, much beyond 45 degrees). Doing that repeatedly irritates my knees a lot.I don't train any throw that require to drop knee on the ground. But I don't mind to drop my knee on my opponent's groin (or body) when he is on the ground.
My teacher used to say "What kind of fighter will you call yourself if you drop your knee in front of your opponent?"