I think it's a great idea! I've had my 36RNB with griddle for 14 months now, kids gone also. If my kitchen were bigger, and I had room for a 48", I would either opt for griddle and grill, or a 24" griddle. In 40 years of cooking, I've never needed more than four burners at once, and anyway the griddle can be used as a french top for two more pans if needed.

You should look at the platinum series. It has an interchangeable griddle and grill so you can have the best of both worlds. And on the plus side, the btu ratings on the platinum go to 25k. So that's plenty hot to grill with.

If you're only considering the RNB, my friend has a grill and feels that it doesn't get hot enough. On an RNB, the grill btu rating is only 15k. For a griddle, that's more than sufficient but you will not get great results on a grill with that btu rating.