Everyone has their limits. For some, that limit is Octave Pro. For some, it's FRP. For some, it's shakers. Or D3. For some, it's Puck. Or the even more egregious lobber levels. One pair of pilots will enjoy and compete hard in circumstances that another doesn't like. This is fine, and even normal. Playing what we like is how we got the great game we have today.
On the other hand, we do need some idea of what it means to be able to compete everywhere 'reasonable', some way to distinguish someone who is missing an essential skill from someone who has a weird specialty.
The best philosophy I know is to play for the respect of your peers, and to respect awesomeness in whatever form you find it. You should never criticize someone's achievements in a level just because you don't enjoy the style, though it does make sense to try to persuade others to play things you enjoy more. And you should never, ever, ever downplay someone's achievements in an area just because you are not good at it. These are dishonorable - literally - community destructive for personal gain. Always respect others' achievements for what they are on their own merits, not for how they affect you - and expect the same.
Playing for the respect of my peers means beating them where *they* think it is meaningful, as much as possible, and defending ground that I think is meaningful. I work to maintain a broad enough game that I have something in common with everyone, and I always try to play opponents in a level we are both good at and which we both enjoy. I seek hard and worthy challenges rather than trying to protect a stat line, and this is not even as losing a strategy as it sounds like - this community is smart enough to judge that for what it's worth. A good record is one path to the respect of your peers, but a less reliable one than seeking out challenges everyone knows are tough and sometimes succeeding.
Greater breadth of excellence means more respect from more people. Conquer and defend the areas you see as worthwhile. And sure - learning one thing may make you better at another. That is the difference in part between an interesting subgame and an uninteresting one, in my opinion - it gives you something you can bring back profitably to the larger game.
We do need a common game for competition to be fair and meaningful - that is what core is for - and I think it covers pretty broad ground, and that going further away from a standard game, in whatever direction someone would like to go, should always be considered voluntary. This is the only way that makes sense to me - I think the only thing harder than picking one level fairly is picking two which are unfair in equal and opposite ways. The motivation for going beyond what is required is not that it is somehow secretly required anyway... it's that there are fun and interesting games to be enjoyed, useful skills to be learned, worthy challenges to be conquered, and accolades to be earned. If these reasons are not at least a little tempting, I'm not really sure what you're here for anyway.
I do think it can be foolish to play things you aren't ready to defend. I won't play ladder matches in levels I haven't played 1v1 before, and I won't let my opponents do it against me. Ladder matches are for games you feel ready to test under high pressure, not games you are learning the basics of. I think if you offer a game somewhere, you are at least claiming that you know enough about it to offer a reasonable fight. There is nothing wrong with training up a game off the record and avoiding games you don't choose to defend. Within reason, it's what you should do.