I havnt read this whole thread, but heres my view.
I think it can be beneficial. I started lessons when i could w/t/kinda canter. So i knew the basics, i'd grown up with horses so I had horse care down pat. I had 2 years of group lessons, then 2 years of private flatwork lessons. It took me ridiculously long to improve as im not naturally talented in riding and i'd just gotten a new horse when i started private lessons, and he was so different to anything id ridden before so it took me a while to get the hang of things. We were also both learning together so that doesnt help.
I stopped getting lessons due to a number of things, but continued to ride. Since then i have improved so much. My seat/balance, my horses way of going etc etc. My instructor was good, no doubt about that, but the time ive had to 'figure things out on my own' has benefited me greatly.
In lessons your constantly told what you need to be doing, when to do it etc etc, and i think i became accustomed to relying on my instructor to tell me what to do. In a way i guess she was riding for me. I suppose i would 'switch off' somewhat, i was also around 15/16 yrs, so the age didnt help!
In saying that though, i can/have only worked on what i know already. I can only build on what ive been taught in lessons. If i wanted to start something completely new i'd need instruction. I can get a horse to Novice level dressage (leg yeilds, 10m circles, lengthened strides etc) but if i wanted to move up (next level is elementary, start shoulder ins, medium/collected paces, counter canter etc) I'd start lessons again. I wouldnt feel confident enough starting these things on my own and it would probably be quite stupid of me to try.
This was probably long and boring, but i think there is so much you cant learn in lessons. Just getting out there and trying new things/riding different horses/different disciplines etc. Talking to different people, listening, keeping an open mind to how other people do things.