these guys hit it on the head with the mulch, but here's my opinion on the fabric:
it all depends on how much traffic the area is going to get. if you plan on leaving it alone, it will work just fine. if you plan on walking on it, or if it is in a neighborhood traffic area, the weight of people walking on it will push mulch chips through the cloth. before you know it, you'll have some shredded, nasty crap in the ground that's not going to do you much good.