Then ask them why they insist on doing it and see what answer you get.
There are different qualities of radiator (thickness of the metal they are made of) - old ones are much thicker. To be absolutely sure you won't have a radiator leak problem that they will feel duty-bound to come back and fix, some plumbers will insist on changing the lot. It also creates more business for them, for course.
Ask the one who doesn't think it is necessary to explain to you why.