Good question Evan. The short answer is that I don't trust myself with knobs. Most of the circuits I mess with have tight voltage requirements (1.2V, 1.8V, 3.3V...). It's too easy to thoughtlessly bump the voltage. Also I figure how often do I really need to adjust the voltage? With a low-cost supply I can set the voltage and leave it attached to the project. That said I haven't ruled out a future power supply with knobs.

These look like a nice addition to a bench, and a low cost way to get a PSU. My concern is with the trimpots used to control voltage and current. I'd really like to see knobs instead... any reason you went with the trimpots?