Some types of brake pads have small metal clips designed to contact the brake rotor surface to create a squealing noise when brake lining wears down to about 2/32" of an inch thick. Many other brands of brake pads do not have any wear indicator clips, so they do not make any noise at all until they are totally gone and rotor damage is occurring. Just because your brakes are not making any noise at the moment does not necessarily mean that they are not getting close to worn out...
For the brakes to function normally the brake calipers must be able to move freely. In some warm weather states calipers can lat for 100,000 miles or more before needing to be replaced. In many other areas where conditions are harsher (particularly northern states where road salt is used in the winter) calipers can become corroded, frozen, or worn out in half of that time. If a caliper has become stuck (frozen) it must be replaced to be able to replace the brake pads and to make the rear brakes function properly.
The shop that is recommending that both calipers should be replaced at the same time is doing the job properly. Calipers should always be replaced in pairs, for a couple of reasons. FIrst, if the caliper on one side has stopped working properly, the one on the other side of the car is the same age and has been operated under the same driving conditions and is also likely to be in poor condition and ready to fail soon. Secondly, a new brake caliper is naturally going to work more freely and easily than one that has been used for many thousands of miles. If only one new caliper is installed and one old used one is left on the car, the new one will apply the brakes on that wheel slightly more quickly and a little harder then the old caliper will. This can cause uneven brake wear, and also can cause pulling to one side while braking. The professional repair approach is to always replace both calipers on the same axle at the same time.
It sounds as if the shop you took the vehicle in to is doing a good, professional repair job for you. If in doubt, you can always take it in to a different repair shop for a second opinion and repair estimate, but if they concur that a caliper is sticking and in need of replacement then both calipers should be replaced at the same time.
I hope this is helpful!