pen cancellation

As for what arrives in Postcrossing, it's not a lot, but what concerns me is pen cancellation. I know that the primary purpose of a postmark is to prevent reuse, so it does its job, but it's just not pretty! I donate used stamps to charity, I'm also interested to see how much of their value is damaged by pen-cancels. But I can't be sure of their value as I just leave them in the donation box. I wonder if anyone has made pen cancellation a postal research topic?