Many are missing the point of the half bath when it comes to a motor home. In the early days of motor homes, they had an all in one bath. You were jammed in there with your knees in the shower while sitting on the toilet. As time went by and motor homes got bigger, people wanted something better. The next generation was the "split" bath. Shower on one side of coach and bath on the other. As great as people thought this was, it had it's own issues. Little privacy and the requirement to shut off the coach while someone showered.
Most recently, people have realized that an all in one bath is really the way to go, as long as it's not a little box. We bought our last diesel pusher because it had an all in one bath, rarely seen in 2005. It was a side aisle model and the bath was quite large.
With the desire to go back to all in one baths, of good size, it had to be placed at the rear of the coach. If you look at large diesel pushers, the majority of those being sold today have a LARGE rear baths.
With all that said, you have to realize that with the bath in the rear, access while driving was impossible, unless you were willing to crawl over the bed. The answer was to install a half bath that could be accessed while the slides were closed while on the road.
So......people aren't buying them because they're snoody or have a great need for a second bath, they're buying them because they're needed to make the more popular rear design work.
When it comes to large trailers with multiple sleeping positions, it only makes sense to have a second toilet, due to more people. In something like a toy hauler, where the occupants are often covered in dirt from the toys, people can use the rear toilet without getting the rest of the coach dirty.
Lastly, we were on a week long trip where we both got the flu for three days. Having that second toilet really came in handy. Remember RV's rarely had a TV in them 30 years ago, but people want to be more comfortable when they travel.