I think it boils down to how much you are out on the road in your RV. Weekend campers may find it is not worth it. Folks that only take a 2 week trip each year may also figure it isn't worth the expense.
The way we looked at it as full timers was it would always be well worth having a washer/dryer. As someone already pointed out it will pay for itself in 2 or 3 years. I have never bought into the argument of them taking up too much space. We don't haul around 12 or 14 days worth of clothing so maybe that is why we felt we had plenty of room for the washer and dryer.