A registration document (V5) is not proof of ownership. The registered keeper should be the person who is actually using / keeping the vehicle and this is not necessarily the owner of the vehicle or the person who is paying for it.
That person is the person responsible for the vehicle so far as official communications from the police/DVLA etc., but the owner is the person who put up the cash (or was given it as a gift).
The DVLA make a point of saying that the person named on the registration document is not necessarily the owner.
This is particularly true with a company car which is owned by the company, however the registration document should show the registered keeper, i.e. the day to day user (this may be an employee who has it as a permanent perk with his/her job).
In the case of a car used by a married couple, ownership of any property is usually classed as joint and if the husband/wife was stopped driving the vehicle without insurance the police would probably accept that he/she was the joint owner and not look to the other partner for additional offences, such as owner permitting no insurance.
A registered keeper will usually be regarded as responsible for parking tickets etc. so it would be wise to have the registration document changed if you are the owner, but not the user/keeper. Additionally, as the owner/registered keeper of a vehicle, there might also be some circumstances where you could be prosecuted for an offence, e.g. if you are permitting someone to use the vehicle knowing full well it is not insured or roadworthy.