Ap Owyen,
I do agree, one never really "masters" a style. But the devotion to a single style can make their tehcnique a lot more natural, and sometimes much more powerful than if they shifted their focus onto other thing. I do applaud RepulsiveMonkey's decision to stick with and attempt to master a single style.
But the key to truly mastering it doesn't just lie in the drills, and training, you also should be testing your skills. I don't mean by streetfighting, or even entering the UFC, but you really should get some exposure to other styles. If you feel that your style is complete, and does have solid principles for dealing with any attacker, the best thing to do would be to find out, by meeting, and practicing with other people outside of your style, and school. I don't mean learn what they're learning, I mean learn how to use your training against their style. You both learn that way.
So again, I do prefer to cross train, but if not, I feel it's essential to expose your skills to other styles to see what attributes/principles in your style you need to develop. It's just that chances are the key to defeating a particular style, lies within that style (ie: grapplers have the best anti-grappling defense, styles with heavy Chin Na tend to have better counters against Chin Na, etc).
It's all a personal journey, though, and I don't feel that anyone is wrong in the path they choose, unless of course they're out ordering CD-Roms, and video games, instead of actually going out and shedding the blood sweat and tears to earn the skills.
Jaguar Wong
"If you learn to balance a tack hammer on your head
then you learn to head up a balanced attack!"
- The Sphinx