Is driving equally safe whether the brake calipers are rusted or not? I found this question, and I didn't read that driving was discouraged with rusted calipers (only that it wasn't as aesthetically pleasing as non-rusted calipers); however, a shop recently recommended replacing them for safety concerns. Some more background: I replaced the brakes ~3 years ago, so should I go ahead and replace them again or are brakes typically good for much longer?
Aesthetics has little to nothing to do with how well calipers function. As long as the caliper pistons do their thing and don't leak, there's no issues keeping them. There are no safety concerns with them as well. Unless the shop can point out a mechanical problem with them, the rust will not pose any issue. You usually won't see anything more than surface rust anyway. This is usually cased from the heat burning away any paint which isn't heat resistant.
There is only one way which rust may interfere with the operation of the caliper and that would be where the caliper slides. This would just entail cleaning and lubing, but not replacement.
In the rust belt, all of the vehicles in our family (GM cars and trucks / BMW / Subaru) will develop sticking rear disc-brakes every year due to rust buildup between the brake-pad plates, pad-slides and caliper-brackets. It seems to make little difference what kind of lube is used; between the heat and harsh environmental conditions, annual service every spring is the only fix we have found. If some offered all stainless-steel rear brake components, I'd be first in line.
I would disagree a little with @Paulster2. Rust can build up behind the rubber dust shield and even the piston seal and cause the piston to stick rather than slide in and out. This will cause the brakes to stick on slightly and cause heating of brake disc and maybe pulling to one side. I have seen this on a number of vehicles, but does depend on the design of the caliper.