Do iPhone battery extenders reduce regular battery life? During heavy use periods, my iPhone 6 battery doesn't last the day. I'm looking at the battery cases available and curious if these actually reduce the life of the resident battery? 

My understanding of rechargeable batteries was that they should be allowed to run down, and then fully charge. But that belief may be based on NiCad technology, and the newer LiOn may not have this issue. 

It seems that if the phone has such a charging case, the case will function the same as just leaving the phone plugged in and 'topping off' the charge.
In theory, no. Battery extenders work just like a charger. The main problem with these extenders, assuming you are not buying some top notch apple certified extender, is the different random voltage they output and that can seriously damage your battery and device.

If you are getting a battery extender I would recommend getting something that is apple certified or at least branded from a known company. Stay away from all the knockoffs AliExpress garbage.