I wouldn't keep a tank without using them. I don't want a sump full of detritus, which will eventually happen if you don't use them. Someone said noise as a problem.......What noise does a filter sock make?
Changing them out once every 4 or 5 days is all that's necessary. I toss them in the wash with some bleach and they come out pretty nice. They cost me about 4 dollars a piece, and I've had these about a year now. Not very expensive IMO