Hi Stan,
I agree that space for a cassette without a shower could be wasted space. For me I would not be without a shower and the wet bath combo with cassette is ideal. My first camper was a NorthStar TC-800 pop-up, it had neither shower nor cassette, but did have the porta-potty.
The only downside I had with the porta was the need for a little more modesty when using it... there is the situation with whom one is camping needs to exit the camper in order to use it. While there is little containment in the wet bath combos, they do offer a little more privacy.