...Why? They're not following you and so you cannot use their combat prowess; they will not be in combat and so do not need new armors.
Weapons that surpass the quality of their originals will always be equipped regardless of whether they follow or not.
It makes for a more polished game is why, doesn't matter that they wont actually be doing anything when you are not in the area, but none the less, they should keep on the armor. When I come home, I want to see them dressed the same as when I left.. sure it pops on once you get outside a city, but.. with the random Vampire attacks on the citys, they need to be prepared for combat at all times.
Just as I want to see Mjoll strutting around riften in her new expensive Daedric armor after we go out adventuring and I dismiss her. No real
purpose to it other then the whole immersion factor.. that and Riften of all the holds is the only one I've seen regular attacks on. She of all needs to be prepared at all times. The house-carls have it easy.. especially Lydia.. siting 24/7 up in my room eating.. w/e Lydia.. I'll take Spunky Jordis over your underwhelming butt anyday.
The bow bug needs fixed too... unless of course if you happened to change out their magical unlimited arrow basic hunters bow to something else, then they would actually consume arrows, then i say leave it as it is lol. I would prefer to have an arrow dispenser right there running along with me over one that uses them.