No real benefit for the average driver, except maybe less air loss over time. If they gave it to me, yea I'd take it, if they wanted to charge me, I'd fill up with air. I wouldn't pay 19.95.
But to take your point
1. Yes from the studies I've read you lose less "air" per month using nitrogen than plain old air, but you still lose pressure.
You can go from nitrogen to air without any problems, but to go from air to nitrogen takes a few step to purge the plain old air out of the tire, this generally involves letting the air out of the tire filling it with nitrogen, letting that mix out and filling it again, until the tire is full if nitrogen. But in many places they just let the air out and fill it with nitrogen and call it good enough.
Pros Nitrogen will mean less air loss over time, and is dryer. Cons, cost.
But don't forget the hype,
RACE CARS use it , AIRPLANES use it, so you should use ti too and for just 19.95 you too can use nitrogen in your car.
Ah, don't look at the reason why race cars and airplanes use nitrogen in their tires, trust us it good for your car.