In your area, absolutely. You're somewhere around New England, aren't you?

How bad is it? Got pics? Personally, I can't see two years of driving
warranting the replacement of rotors for rust.

What /can/ happen though, is that you'll get rust on the friction surfaces.
This gets buffed into a glaze, reducing the effectiveness of the rears. The
effect is to throw more load on the fronts, accelerating wear, and possibly
leading to overheating (and warping) of the front rotors.

Those rear rotors will rust like crazy in a short period of time. I always
thought rear discs were a dumb idea anyway. Too much trouble.

No, not "slide on new". You MUST first sand off ALL the rust on the hub,
and make sure it's super clean, or else you can get some serious vibration.
Use 50-grit emery cloth.

And if your parkoing brake is the kind that's a small drum inside the disc,
make sure you move the parking brake lever to "off" before removing the
rotor.

I got my 03 pilot back from the dealer - one item puzzles me - 85k miles on
it and they said I really needed rear brakes - they wanted $500+, but I only
went for rear pads - $162.  Is it normal for the rotors to rust on the part
facing away from the wheel?  I will check how much to replace them on my
own - are they like my 94 civic, move the caliper out of the way and just
slide off old and slide on new?  I would guess $150 for the pair of rotors?