The driving other cars extension is ok IF the car has its own insurance AND you are OK with the risk of damage happening to the car while you have it.

A) Because if the car isn't insured in its own right it won't be insured while you aren't driving (eg while you park outside the papershop or something) and unless I'm mistaken it would be illegal to leave an uninsured car on the street. Also, how would you buy tax for an uninsured car?
B) Because you only get third party risks covered so if the car is stolen, goes on fire or is in a bump, there is no payout for your "mates" car.

Personally, I don't approve of the any car extension being miss-used because it is already harder to get and could disappear completely.