Why not? If you are getting outs with it, go for it.
Phil Niekro had an excellent sinking fastball and slider for many years. His brother Joe had both, and a curve too. Tom Candiotti had both fastball and curve. So chris is right, there is a prescendent.
When to use it? hmmm, Well first off, when no one is own base seems good. Maybe as a good firstball strike, as you fastball will look very quick after the knuckle wafts in. Use your curve as a test:If you curve is not being called for strikes, you knuckler probably will struggle as well.
If its working by all means keep it. Uhm..you might take you catcher out for a nice dinner every once in while too, Ian,