I find cabinhost's view a bit cynical. There are many reasons that people choose an AirBNB over a hotel, with cost being just one of the factors.
In the past, when I have had requests for special items, I have offered to split the cost with them. A bench in shower is something that MANY people enjoy. Women shaving their legs, men who worked out too much and want the warm water to relax muscles, toddlers so they don't get sprayed everywhere.... if your bath would benefit from a bench suggest that they help pay for it in case no other guests decides to use it.
And for all three times this has come up, the guests were thrilled to help with the cost and my reviews from them were stellar.