You're correct that the tensioner does eat away an indentation in the button. I have an example I can take a picture of at home.I read once, and I’m not sure if it is correct, but someone said, as the stock tensioner foot cuts into the tensioner button, it then applies less tension to the chain, allowing more slack, and thus changing the timing slightly. Don’t shoot the messenger, I read that though, not sure if it is true. I would think the spring loaded tensioner plunger would just put more pressure to make up the difference, but maybe not.
The main issue with the spring plunger is that its just not strong enough in high load situations.