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.
He 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 was stopped driving the vehicle without insurance the police would probably accept that he was joint owner and not look to the wife 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. There might also be some circumstances where you would be deemed as being the owner of the vehicle for an insurance offence, e.g. if you are permitting someone to use the vehicle knowing full well it is not insured or roadworthy.
Most insurance companies insist that the person who they insure is the primary user of the vehicle and can specify that the person is the registered keeper. It is up to them who they will or will not insure.