Bands are easy to carry, easy to set-up once you know how, and if your gym doesn't have them they are relatiely inexpensive and can be carried into the gym easily, or stored easily in your home gym. Dragging 200 lbs of chains into the gym is a drag if your gym won't let you leave personal equipment there.
That said, it is generally recognized that the correct progression is:
Weights alone
weights chain
weights band
weights band chain
The bands are MUCH harder on the joints and connective tissues and most people tend to use too much band weight for too long a time period.
If you decide to use bands, never, or rarely do the same lift more than 3 weeks in a row, and don't do a large percentage of your lifts using bands. They are a wonderful tool if used correctly, and a great way to overtrain if not used correctly.
Iron Addict