You guys are right, pinning someone, AND having tools to fight with at the same time is pretty much impossible. But I was refering to someone taking you to the ground, into there world, and not having the reflex time (which can be improved by sparring) to stop a takedown, or at least roll it. Sweeping is also a good skill to have. a few of those "elite groundfighting" skills would come in handy. especially if you dont want to maim the person.
BTW, I said "ground" twice, and "groundfighting" twice.
I love Taijutsu, but, like any martial art, there is things missing that could come in handy, like sparring, and sweeping.
Also, there is a ground (wrestling) technique called saturday night ride in which both legs are hooked, the arms are hooked, and the head is, naturally, close to the ground. headbutting from the bottom on a hard surface could be dangerous, and the person on top could headbutt and bite easily.