What's the argument against putting a toilet (stool) in a shower stall?

Since the toilet paper would be getting wet, you are saying you would just clean yourself with soap and water?

Yep. Some RVs, as well as passenger train private baths, have a shower head on the ceiling and when you shower the whole bathroom gets wet.

When I lived in Thailand our whole bathroom was tiled and had a floor drain, with a shower on one wall and toilet at the other wall. The toilet got splattered when we showered. The TP was far enough away that it barely ever got splattered.

If our bathroom in N Cal had been designed that way, our water leak 2 years ago would have cost us only $100 of water instead of >$50,000 of repairs.