RockJock-- I believe the OP's problem in both cases was the oring on his reg. That would cause leaks with or without the yoke adapter.
Spectrum-- What situation could you be in where you need to change the oring, but don't have your gear with you? I don't take my regulator off the tank when I'm in/near the water, even for shore diving, I'd be walking up the beach out of the surf. Then, I'm gonna want to dry off the oring and make sure it doesn't have sand or salt on it. Besides, most gear leaks occur when setting up equipment, and should be noticed long before you move away from your car/gear/save-a-dive kit. First thing I do when setting up is try to get regs on tanks and turn the air on to let the stuff sit, pressurised, for a while, so that if something is gonna leak, it'll do it then. At the start of a dive is when there is the most pressure on the components, and as that pressure drops, the chances for leaking drop. If you want to keep one oring on your tank, that's fine, but I think a better location is in the save a dive kit. Plus, I hate hate hate trying to take off those silly tank cap things which always seem to have too short line, or knots in bad spots, or are all twisted up... they are worse than Chinese finger traps. I once tried to change a tank oring out for one that was slid on the line of the valve cover thingy, and after a few minutes I gave up, walked back to my car, got out my keys, pulled out my kit, and got a new oring.
Now probably I'm just a butter fingers, but I can't be the only one!