I do (well, I can, and will when I'm out of 12 grams). I use a remote though. My workaround to minimize the awkwardness of the remote was to make a "gas-thru" grip for my Zeus. This way the nipple for the remote sticks out of the bottom of the grip and it doesn't really get in the way when you're using the gun OR when you have it holstered.
I haven't gotten a chance to do a writeup on it, and I don't have any detailed pics, but you can kind of see it in this pic if you look closely:
My biggest beef with a tank + bottomline setup is the fact that it makes it almost impossible to holster the pistol. Now, for a lot of people, if they're running pistol-primary they almost never holster their pistol anyway, so if that applies to you, then no big deal.
I play 99% scenario ball, and have taken to running missions and flipping flags. Sometimes, a prop is too big to try and carry one-handed and I need to holster the gun. Or, I want to try something devious and sneaky, so I holster it in the hope that not having a gun in-hand will make people view me as less of a threat or think that I am not in play. Hell, it worked with my DSG in the huge scabbard on my back, as I dead-man-walked right through the main firefight at the center flag as Dark Ops Extreme 5 and went to run a mission in the other team's backfield.
The other thing that bugs me about is it the balance vs. shots per tank. I use a 9oz. tank so that I can get about 500 rounds (which is a little over two reloads for me, carrying about 210ish rounds on me) out of a tank. While small for being mounted in a pouch on my belt, the 9oz would be way too big and awkward on a pistol, too short to shoulder and too long to not interfere with the balance. a 3.5oz would probably be ok, but then I'd have to carry spares into the field to feel safe about not running out of air and its one more thing to worry about. So that's why I went with the 9oz. and the remote line.