Cheap, disposable, don't take up a lot of storage space, easy to get rid of discreetly.
I don't use them but would consider it in some situations.
Just FYI, don't put your plants in bags until they are going in their ultimate resting place- moving bagged plants jostles and can damage the roots. If you want to move them, check out Psycho's grow log- he moves bagged plants by placing them on cheap houseplant caddys on wheels.
Another suggestion on grow bags is to use them on a grooved drain tray (like an ebb n flow table, check the Botanicare ones for what I'm talking about) or on a grate, because a grow bag just plopped on the floor doesn't always drain so well...