Some help for a noob

[lfd]

Are there warrior skills that are only usable in certain stances, and that is why switching stances and Tactical Mastery are necessary?

Yes. Also, if you're tanking you're most likely in defensive stance, whereas for high damage asskicking you're probably going to be in berserker. Whereas some skills can be used in multiple stances, some are tied to a specific one.

So far, I seem to only have skills that work perfectly fine in Battle stance. Is it maybe that they are simply more effective in certain stances?

Not the skills, per se, but the warrior ;-)

Also, any warriors here use any add on programs, like an enhancement for the interface? I'm thinking of using something that will add another line of icons on top of the existing one...

I use ctmod to move stuff around, but you can get an extra bar (two, in fact) without add-ons by turning them on in the interface options menu.