Nitrogen is not expensive. Just buy your own tank. Nitrogen not only has larger molecules so it doesn't leak through the liner as quickly as Oxygen which has smaller molecules, it is also dry and retards corrosion inside the tire/rim assembly.
Aluminum and magnesium are subject to oxidation corrosion (which means reacting to oxygen).
Nitrogen is non-reactive.
Locking stems are required in case of the tire rotating on the rim which if tube type would want to rip the stem from the tube. Bead locks/screws are a great idea no matter what type of tire you run.
On a street application it is not recommended to run a tube in a tubeless tire because the inner liners are different.
A tube type tire is constructed with a smooth inner liner to minimize friction/heat build up between the tube and the liner. Also normally a Talc like substance is used between them.
Tubeless tires have ribbed, textured liners which are not compatible with tubes.
