a reservation, or priority seating as they call it, is important if you're going at a busy time of year or a busy time of day. We're going in June and want to eat at 6:00 PM on the night we arrive so a reservation seems a must.
When we have gone to RFC in the past around these times the waits were long and we were glad we had a reservation. The addition of T-Rex has probably split the demand between the two restaurants but I don't fell like chancing it.
To, Trissy, the restaurant was walk-up only for the first couple of months. They didn't start taking reservations until January.