Some people use them occasionally. I don't recommend it personally as they lack the nutritional value required. You'd be a fool to feed your snake soley on chicken necks or drumsticks, but it is ok once in a while either as a treat or if temporarily you can't get your hands on 'real' food..
If you have a steady supply of rats then there is no need to be searching for alternatives. Rats are perfect and contain all the nutritional goodness that is required to keep your snake happy and healthy.
