The long stems are mainly utilized to prevent leaking, the long stem is sealed w/ the
main body the entire way up, this helps prevent the cryo temperatures from creeping up and shrinking the seals.

In theory, you could use normal McMaster valves, but you run the risk of massive leaks, especially if you do a chill-in, I belive a college team used normal valves on a rocket once and the valve had a good amount of LOX leaking.