This pretty much sums it up. There's about ten other things that are self-evidently more important in a job search than this.

A long time ago I spent a little while debating whether listing my middle initial on my resume would make it look "more professional"

Just watch it with the full middle name, which has a serial killer ring (jk (mostly)).

Maybe true in most cases, but adding a somewhat niche skill set to my Linkedin profile late last year almost immediately yielded me an attractive job offer.

Unfortunately that's the appeal (and basically the value proposition) of linkedin - it's social media dressed up to make you feel like you're doing something, when you're really just polishing a turd (as it were). Anything is better than spending time there, if you want a job or clients or whatever, but it's the easiest thing to do so people keep coming back.