It will make suggestions on what keys to roll your face over next to maximize your survivability.
It may teach you a thing or two about priorities and what to use when and will quite likely greatly improve the fluidity of your gameplay.
Other than that..
Been doing dungeons since I hit level 15 and never had any issues. Sure, if you don't pay attention to what is going on you will likely die once or twice. But that is more or less a non-issue.
Nowadays I faceroll through heroic dungeons like there is no tomorrow, put up with them "nice" people in LFR and may eventually give proper raids (No, LFR is not a proper raid!) a try at some point (small guild is small).
BrMs are totally viable for just about any tanking situation you can possibly think of.
I do have to say that bringing a healer with you who knows how to handle BrMs helps a LOT :)