Many people own more than one vehicle, whether a summer plaything, through necessity (eg they live in an area with no public transport or need emergency 24/7 mobility and must have a spare vehicle if one breaks down) or because they have a hobby where they collect or like to do mechanical or modification work ("enthusiasts") – and other reasons.
If one needs or wishes to use more than one of one's vehicles (of the same or differing types) at once, or even just might do, then one has to pay for multiple VEDs, even though only one is being used at a time, only one is causing road damage, carbon emmissions or whatever other purpose you see the VED existing for.
This imposes additional and unfair costs on enthusiasts, those with unreliable vehicles, those with (say) a hobby of motorcycling but a need to use a car to travel to work or carry the family around (or vice versa, perhaps): and similarly those who have vans or larger cars (SUVs, people carriers etc) for commerical or practical purposes but have a smaller can or motorbike for personal or city use.
You cannot even test a vehicle which has been off the road for a while, undergoing repair or storage, without paying the VED for a minimum of 6 months. You cannot take a vehicle out even for one day a year without having to pay the VED for at least 6 months, whether or not you want to use it again. This is unfair, and only leads to VED "evasion" and ill-will against the authorities who levy this duty and enforce it..
If you want to encourage people to use the right vehicle for the right time and place, to protect the environment for instance, then don't discourage them from using whichever vehicle might be best, just because it would have to be paid for and "taxed" to be used.