A shop has to know how to diagnose and repair problems across a wide range of bikes. If you come in complaining about your brakes the shop needs to fix it for ~$50+parts, which means about an hour to diagnose and repair. Just for brakes there's canti, V-brakes, dual pivots, mech. disc and hydro-disc all in common usage. I would expect a shop to fix problems with all of them.
For esoteric things, the enthusiast has the time to learn about how to properly fix it. The example Lefty fork is a perfect example. An enthusiast can spend 10 hrs carefully rebuilding his fork over the weekend. For a shop, that would mean something like a ~$400 labor charge, which no one would pay.
I do almost all my own work, but it's good to recognize when a shop is a better option. For regular repairs, it's worth it to me learn how to do something, because I'll likely do it many times. For some things, the LBS is a much better option, when learning to DIY myself isn't worth the time & effort. I paid a LBS $30 labor to convert my MTB tires to tubeless (non-tubeless rims) for the first time (installing tape, sealing, etc). It was worth it for me because I could rely on their experience to pick and install the proper things needed for the job, and if it didn't go smoothly it's their responsibility to fix it. Doing that job myself for the first time, is definitely going to take several hours. Once they were setup, I knew that I could handle installing new tires, adding sealant, etc. Odds are I'll buy a new bike before I need to change the basic tubeless setup.