Blue badges are issued to an individual, many of whom do not drive themselves, have a car or any other from of transport.
The badge is for the use of teh person and is not tied to any car like the disabled tax disc is.
If a family have two cars, do they have to always go out in the car with the disabled tax disc? no, of course not. Also the fact that they are being driven round by a reletive/friend/taxi means they still need the disabled parking spaces or being able to park closer to the place they need to be.
Of course, if they are not in the car i.e. they are not being dropped off/collected etc, the badge should not be used and be taken away....