How much does writing on cards hurt their value?

[wdwfan]

I see a lot of cards from the 60s on back that have writing on them. For these, it seems the owner's initials were AA because that's what was on almost every card. Some of them have the team name crossed out and another team name written in its place. Why someone would write something on cards that were worth so much (or crease them up or put tape on them or put them in bicycle spokes or whatever). Heck, I was looking closer at the Frank Robinsons (and a couple of others) and there are tack marks. Like someone hung them up on the wall or something. I can't understand why people didn't take better care of their cards back then.

Originally Posted by Jim65

To me, intentional damage like writing or trimming is a big dealbreaker unless the card is super rare.