Recent music festival with wife and daughter (pic below).
Crowded! Literally inches between our van and the neighbors on either side.
Now add rain the last evening, and the corresponding mud fields.
Now imagine the inside of the porta potty, never mind the trip there.
Suffice it to say that having a bathroom in the van was really nice..
On a related note, VERY happy with the cassette toilet option too. We just dump into the porta potty, or in the past at a road-side bathroom or even our toilet at home. No dependency on dump stations, and no messing with black-water hoses or valves.
One note related to cassette toilet: I did have a problem at one point and broke a mechanism outside the cassette itself that opens/closes the top. This was easily resolved (better than new now), but not before I dropped the money on a new cassette which I am not sure I needed now.
I think the mechanism is weak by design, and a slight misalignment on the open/close lever at the toilet at the time of inserting the cassette can cause it to break off without even knowing. The effect is that the toilet will not close after that. Thankfully, it was upon inserting an empty cassette, so the smell was not horrible.