Thats interesting. I currently have six laptops shelved due to bad batteries. Two of those are macs. They have all been solely used as desktops, only moved a few times. All had their batteries fail around or under 2 years of use.
That said, i do admit that the batteries of Macbooks can usually take more punishment than Wintel machines.
Aslo, i refurbish coporate computers for a voloneer organisation. 80% of those used office machines were docked 24/7 and in my experince those that were used properly had batteries that survied longer. Around half uad dead batteries after the three year life span. And those are all thinkpads and busness dells or hp's. So no cheap ****.
If you're refurbishing computers then it sounds like they might be a little older and might use lithium ion cells, vs. lithium polymer, so that might not be relevant.
Also, if you overcharge a lithium battery, that will quickly ruin it. Older laptops didn't have as much logic around preventing overcharging so maybe those Dells and HPs were ruining themselves when they were plugged in by trying to charge the battery all the time.
You don't give much information about the Macs.
You are not the first person to claim that leaving a MacBook plugged in will kill the battery though. Since I leave my MacBooks plugged in all the time and it's never been bad for the batteries, I have a hard time believing this.
I have put a little thought into trying to come up with alternate explanations. One is that if you leave the MacBook in the same place all the time, i.e., use it as a desktop, it might cause problems. That place might not have very good airflow/ventilation, so the laptop might run hotter than it otherwise would, and we do know that heat is bad for batteries. Or, similarly, that place might be exposed to a lot of direct sunlight (maybe at times when nobody is around to notice) and that could easily cause a heat problem.
