Some of my first quilts I backed with broadcloth ... and pretty quickly came to regret. It just doesn't behave like cotton does - the added poly made the quilt too hot (even in Canada, eh!) - just doesn't breathe like the cotton does.
But really, it depends on how you will be using the quilt - lots of utility or donation quilts will be just fine. The ones I've worked on going to missions for the homeless are made from a complete mish-mash of fabric.