The theory is that bias binding will wear longer because a single thread in the fabric doesn't run across the length of the binding strip. (If that thread were to break it might cause fraying and compromise that edge of the quilt.) However, bias is harder to apply because it stretches. I don't use it on my quilts, but if I were making something that I wanted to pass down as an heirloom, perhaps a hand quilted whole cloth quilt (like that's gonna happen!) I would use bias to bind it.