Nowadays, there seems to be a couple of possible reasons to use a full-service broker:
1. Cluelessness. People who know nothing about investing and need a ton of hand-holding...and don't realize that they can still get a lot of info/help at a discounter...or are afraid to try it.
2. Being filthy rich...to the point where the full-service broker is going to pay a lot of personal attention to your account(s)...and probably give you all the best rates.
One gray area is where you don't have a choice...like the company 401k is with Fidelity, so the commission is $17.95 and not something lower. Of course, for Fidelity, that's a discount rate.