It should be rare, and we should not be able to repair it, not without the knowledge to do so. PA is supposed to be so complicated BoS scribes spend their whole lives learning how to repair it. Only a small number of really smart people should have the know-how to fix it and it should cost alot. A wasteland junk dealer should not know how or have the tools to fix it.
I agree with this, but I feel that, at a certain point, you
should be able to repair it. First, maybe you'd have to find someone who knows Power Armor extensively (after you get the perk [assuming you need a perk] to wear it). You would start out by just 'watching' the person repair it, then you would start to help in a few minor details, until you could do it all yourself. I.e. you would have to 'learn' how to repair it fully. After learning, you would only be able to repair it at a workbench. Possibly, if you continue to study, you might be able to figure out how to repair it in the field. But I think that
after you are able to wear it and/or able to repair it you should be able to loot it off enemies.
And I'd love to see a system where you could 'add' items onto your PA, like additional armor (makes you tougher, but slower), or helmet mods or something.