Aside from the smoke smell, equipment from a smoking home accumulates a layer of nicotine that discolors the unit and clings to the electronics. It is not impossible to clean, but it is not easy. I used to hand out in an audio repair shop with a friend who is an engineer and repair tech. The pieces that came in with nicotine residue were easily recognized. It's not something you would want on your purchase.
Should we care if a piece of equipment comes from a pet-free, kid-free, smoke-free home?
I agree that I do not want a preamp or speaker coated with a film of smoke, but I am confused as to what a child or pet could do that would not be otherwise visible. You would see the cat scratches on the speaker grill, the dog chew marks on the rack, the iguana turds in the amplifier cooling vents. And what's a little dog hair among friends? As a 3 year old, I supposedly put my finger through my grandfather's new DIY speaker but I'm pretty sure he replaced the drivers so even that doesn't really matter. Most things will be visible and/or easily remedied. Maybe the price will even be a little lower because of the cosmetics. If it doesn't mess with function, not a big deal to me.
What concerns me more is does it come from a drunk-free, drug-free home. I think befuddled owners are far more worrisome than pets or children.