Pretty much the only reason to get a doctorate in clarinet is to teach at a university, which is what I ultimately want to do. It does have the happy effect of polishing your playing to a higher level than when you started, but depending on where you go to school (I'm at Michigan State), it can actually leave you less prepared to play in a symphony than it otherwise would, since most doctoral programs don't have the single-minded* focus on orchestral performance that you'd get at a conservatory like Julliard (where our very own Steven went, if I recall correctly?). I know that I personally don't stand any chance of ever playing in a top-tier symphony, and even smaller regional ones would be a longshot.
The world of instrument construction and design is small enough that there isn't really a standard path to a career in it, but training in instrument repair seems to be a relatively common element among the big-name folks. I guess that makes sense