I find it very useful and essential. My 16-month old boy has been on cloth diapers since birth, and just started trying training pants. He is already potty-trained for poo, still learning for pee.
Cloth training pants enable the child to feel the wetness and aids faster training. It absorbs one pee, so keeps the floor dry (as compared to wearing only an underwear).
From my observation and reading, children using disposable diapers full-time may develop diaper-dependence. When that happens, it takes a lot of effort to potty-train them. I know a few who need diapers at age 3-4. On the other hand, using feel-wet cloth training pants/diapers can help potty-training by about age 2.