Whatever air is inside you marker when you unscrew the bottle will be lost - there's no way around that. However, you lose a little bit of extra air from a pin valve because it has to close as you unscrew the bottle. An on/off valve can be closed before you start to unscrew the bottle, so no extra air is released. The amount of air saved is relatively small, as you still lose the air in the marker either way.
More important perhaps, is that the on/off valve gives you more control over the air. You can make your marker entirely safe by closing the valve and firing a few shots to use up the air in the marker - no need to remove the tank to close off the air and make your marker safe. Pin valve can also jam open on occasion (SP on/off valves can have this problem, but most don't) and can be harder to fill.
As has already been stated, if your bottle doesn't have a leak, it won't leak. If it does, it will. If the bottle has a perfect seal, where can the gas go? It has to stay in the bottle.
It's just like a battle zone, you got a bottle and you're on your own.
Gotta be a Scotch or a Kamikaze, none of those other weapons will faze me.
Hundred, hundred, hundred bottles on the wall, you wonder if you can drink them all.
Got to go home by 1am, the bottle wins the battle again.
Husker Du - "First of the last calls"
Back when I was popular
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HustlePaintball.com for Spyder paintball guns, Dye Markers, WDP Angel 1 and of course the Hustle Hotties.