I have on several occasions extracted skills that soon afterwards I realize I need for something I frequently do, like fly a certain ship, use a certain mod, etc.
I would like the option to lock / unlock skills so that this doesn’t happen when I’m extracting skills.
I think this is less an issue of skill locking/unlocking and more of an issue of skill planning, for which tools like EVEMon exist. Locking/unlocking skills would be an obscure feature no one would use and would only address the symptoms of the real issue, which is better planning in advance.