This comes up often.
Common sense applies in many ways. When my wife (the benefit recipient) could still drive, we ran two cars. One Motability car (for her) and one private one (for me). When her disability worsened and she could no longer drive, we went down to one car. My private car was far better than anything available through Motability, so my preference would have been to retain it and ditch Motability, but my car wasn’t as accessible for my wife or her wheelchair (it was a saloon). I was happy to buy a different car privately, but my wife preferred to stick with Motability.
I called Motability to explain the situation, that we only needed one car as I was now the only driver and obviously there would be frequent ocassions when I would use the car on my own and not always for her benefit. In truth, I was rather hoping that I could say to my wife that they’d said that this was unacceptable and we’d have to leave the scheme, but the complete opposite was the case. They said that the rules are in place to deter blatant misuse and enable them to recover cars that were being abused (e.g. named driver lives miles away, rarely sees the benefit recipient and uses the car as their own), but that a couple who live together and only have one car can use the car in exactly the same way as they would if they had one private car between them. So that is what we do and will continue to do without any concern whatsoever. If it wasn’t acceptable, then Motability would need to recover tens, if not hundreds of thousands of cars from households where the Motability car is the only car, including from many contributors to this forum!