Motability have to make the comment about it being for the disabled persons benefit in order to comply with their block VAT disability exemption. The reality is somewhat different though.
There will be hundreds of thousands of customers where the Motability car is the only car in the household. We are one of those households. My wife can no longer drive so it would be utterly pointless having two cars. There are plenty of times when I use the car without my wife with me and not all of these journeys could be deemed to be of benefit to her (although she might say that having a break from me IS beneficial 😂).
As this topic comes up regularly on this forum, and some people are very insistent that every journey must be directly for the benefit recipients benefit, I called Motability to discuss our situation when debating whether to stay in the scheme or leave. They reassured me that the policy is designed to avoid blatant abuse (such as a family member getting the car through someone else’s allowance but the user lives elsewhere and rarely, if ever, takes the disabled person out in it) but in the case of a couple living together with one car you can use it in exactly the same way as you would a sole privately owned car. As @Carmad rightly says, this can even include the spouse of the disabled person using the car for commuting.
So, in your scenario, you can drive the Motability car whenever you like and you will be covered under their insurance (as long as you are a named driver).