It is a cheap ineffective way to placate women who are in abject pain
I love gas and air, it completely removed the pain of contractions for me. Its big advantage is that if you try it and don't like it then it doesn't linger in your body.
G&A I thought was like the level of drunkenness you have after a single small glass of wine, just slightly removed from life but knowing it wouldn't last long. Pethidine I hated, it was like the level of drunkness you have when you've already vomited and you desperately want to feel better but know you're going to be lying on the cold bathroom floor for hours until you sober up enough to get up, brush your teeth, get a pint of water and go to bed. That makes me sound like an alcoholic, I actually have a very low tolerance for booze and it has been years since I felt like that!
Anyway, while the fear about increasing areosols sounds reasonably sensible cross contamination dosn't. The mouth pieces are disposable and the system is pressurised so the gas comes out, nothing goes back into the system.
My third DC came very quickly and so I had no G&A. It was OK, a first labour is different to subsequent babies, you know what you are doing and mentally can manage the pain better. It's like running a marathon, the more you do it the easier it is mentally.