I play NL holdem only.Well not only,just 99% of the time.Guess I'm a one trick pony at best.
You need not play all the others to sharpen your skills at NL.Might make you a better HORSE player,and maybe even a better overall poker player,but not a better NL player.
For example look at Phil Hellmuth.He is one of the best NL players there is(and he'll tell you that himself).The fact is he is not very good at all at other poker games.
I think Doyle said it best :"No-Limit Texas
holdem is the Cadillac of poker".There is the perfect amount of information for a good player to win over one who is not so good.
I in no way am against other poker,it's just not for me.All of my time invested will be in improving my NL holdem game(cash and tourney).That is what I believe will make me a better NL player.