the potential damage to the battery of an MBP with a MB charger is as i said, it is easy for a high powered MBP to use more power than the macbook charger can handle which causes the computer to draw power from the battery instead of charging it. Not only does it charge slower, but the constant draining and charging in a short period of time, especially if you're doing something cpu intensive can damage the battery.
with a higher power supply, like the 85w meant for charging the MBP, drawing power from the battery can be generally avoided.