Using a TA for domestic travel? Almost never. However, some can definitely come in handy for international flights, especially consolidators. They can indeed get you fares that you really can't get yourself. Compared with published fares -- these can be HUGE savings.

Although there are exceptions to every rule -- I have rather unpleasant memories with Council Travel when I was in college. I needed to get to BKK, and each time I asked for a fare quote, it would literally take 30 minutes of typing on the other end of the line before I would get any sort of response. And the routing they suggested? SFO-NRT-ITM (Osaka Itami), spend the night, KIX (Osaka Kansai)-HND (Tokyo Haneda)-transfer to NRT within 1.5 hours - BKK. Could we skip the whole Osaka round-trip and just stay in Tokyo? No. Can we earn miles? No. It wasn't even that much cheaper. Thanks, but no thanks.