Why wouldn't you use argon?

Yep, that's what the guy in this thread did. He mentioned it a few pages into the linked thread.

Argon would work. I just thought I remember some Ti welders I knew mentioning they like to use helium for Ti. Helium is not reactive at all while Argon can be slightly more reactive due to a larger electron cloud. As in it is easier for other elements to pull away an electron from Argon which has a larger electron cloud (where electrons are farther away from the nucleus) than it is from helium which has a much smaller one.

But from a macro level such as welding a Ti frame I doubt it matters much.