Simple fact is whether it is left on the charger most of the time, all the time or you use the battery most of the time, as long as you don't abuse the battery by letting it run downto zero or very near zero, the battery will eventually wear out and will need to be replaced. Batteries only have so much life span in them. Some longer then others. But using the notebook always on AC power will not extend or shorten the batteries life span to any great degree.
The object it to use it and not abuse it and you will get the longest life span out of the battery for that particular battery. Again some longer or shorter then other batteries in that same model Mac notebook.
Here are two basic battery article for the MBP:
This article describes the situation where your MBP is attached to a power adapter and stops charging the battery after it is fully charged and then resumes charging when the level drops to the 93%-95% level.
As you can see, this is designed to do that and is normal.