well, I can't argue with what you believe and what you've been taught.
back on topic...
do you know how the rest of the phase goes, "Jack of all trades, master of none...."
the full phase is: "Jack of all trades, master of none, though ofttimes better than master of one".
people forget that part.
I believe diversity in any Art allows a person to see aspects of the wider view of their Art of choice. you need to look outside the artifical boundries (styles), in order to make the Art your personal own.
That philosophy follows thruout history. Samurai studied shodo to great depth...the parallels, if only in mindset, to mastery of sword and brush are well established.
The concept of niche 'styles' are more accurately mostly political and economic divisions.
nobody can really tell me, for instance, the fundamental differences between matsubayashi, kobayashi and shobayashi with other homegrown types of 'shorin ryu'. I've heard traditional dojo say that 'jack of all trades' phrase and drill into people that sticking with one was the way to go. I question what that philosophy is based upon other than student retention. Then within the same class, see them incorporate things that I know came from their alternate training. perfect example is someone learning a traditional art and all of it's kata, then later incorporating jujitsu techniques for the 'bunkai' of the kata which they might have picked up over the years at seminars and whatnot - which is fine, but don't tell me the anti 'jack of all trades' bunk. or worse, make stuff up when they are at a loss to explain the technique due to their extreamly narrow experience of the martial arts in general.
one style can't train it all...and neither can one person. but it's a fact that certain dynamics and economies of movement derive from other arts...by looking closer at those tangental arts, I emphatically believe it gains insight into our art of focus.
name anyone who started a ryu during the 20th century, and you'll find that they studied more than one Art prior to starting their own system. often times even simultaneously training in multiple arts. by your definition, they are masters of none.
"Do not seek to follow in the footsteps of the men of old; seek what they sought".