Hi Gus. Since your question is should they EVER replace dumbbells, I would answer “yes,” since I can think of many reasons why you would substitute the bands for the dumbbells. For example, for a beginning, in-home exerciser where the trainer cannot easily transport weights (for example, I recall one trainer on this site who stated that she performed in-home training using public transportation) bands CAN be an adequate substitute for dumbbells. So, where cost, space, or the ability to transport the weights are an issues, I would say that bands could serve as a substitute.
That being said, I’m not sure that you would want to EXCLUSIVELY use bands with a healthy client who has the ability to progress with their strength. Eventually, introducing weight such as dumbbells makes sense for a progressive way to improve their strength.
I hope this helps.