Od,
I will say this. I have played with a lot of league players, and because of the nature of league systems, the top of the heap (i.e. 7's in 8 ball, 9s and 10s in 9 ball) are often far far beneath shortstop level players.
An instructor will simply allow you to attain your highest level of skill, allowing for the time and desire you have to improve. You are high enough in your league that you obviously have both desire and a good bit of time. That being said, if you are not running multiple racks quite often on a 9 footer, then there is room for improvement. Depending on where you want you game to be.
In my practicing days, i felt I was close to breaking through to the level where I was a threat to run multiple racks any time I stepped to the table. With some work on improving my break, I was easily capable of controlling a 7 game set. I did this without an instructor, but the process would have been SO much faster with a good instructor. My fundamentals were solid, but I was shooting some position shots the WRONG way. I would use inside follow instead of natural two rail angles. And I was still running out fairly consistently. An instructor would have corrected me, and probably improved my game a ball or two overnight.
Remember, just because yer getting out doesn't mean yer playing good.
Russ
Although I hear all kinds of stories and claims that's one that if he didn't actually say it he surely could of! Hey when are we going to play? you better catch me before the 20th I am getting tuned up by Randy (just a little warning buddy! 
true!