Last year, I built a One Ocean Kayaks Cape Ann Storm LT, with a skeg according to Nick Shade's plans. I had never used a skeg before, and I found it useful with a tailwind.
I built a slightly modified Outer Island three years ago. I reduced the overall length 5%, but used the same rocker measurements, in effect giving it more rocker per overall length. It still tracks like a bullet with no skeg.
I'm building another Outer Island, this time to design length, and planning to put a skeg in, mainly "because I can".
In answer to your question, there are kayaks that need a skeg, but the O.I. is not one of them. Put it in if you want, the weight increase will only be about 1-2 lbs. Also consider the low rear deck of the O.I-even though it is compact, Nick's skeg design will JUST fit. The skeg box will also present a significant obstacle to stowage in the rear compartment.