I would of said you would be asking for trouble driving the car with a fault like that. It will only get worse and you'll cause more damage and possibly break down/overheat.
They are correct in saying the headgasket isnt guaranteed to fix the problem but it is the most common cause of this type of fault.
Thats why i said to crack test the head aswell. A hair line crack in the head or bores of the block could cause the same problem but is much more uncommon.