When you're out looking for an RV many people say that it must have at least one slide. Some go with as many as four. Then there are the people that don't mind not having a slide at all.
Some things you have to remember when buying a RV with slides are that the campsite has to be extra wide to facilitate the extra width of the slides. One slide adds quite a bit of extra room and when it isn't open you only have one additional wall to put up with. Which will allow you easy access to the rest of the unit from the front. The more slides, the more walls making for more obstacles to put up with trying to gfet to the rear of the RV when they're not opened.
The people looking at them without slides just don't realize the extra space they're gaining so they figure they don't need it.