I consider it a valid topping. The ham and pineapple combination is something that is not completely unusual, even in the Philadelphia area, where pizza is strongly scrutinized.

Around here in Philly, there is a large grouping of Italians and those who copy their style, which is sometimes also called New York style pizza. There is nothing wrong with having a pineapple slice. I'd prefer a pineapple slice to be one of many varieties I'd have in a single sitting. Perhaps, a plain slice, then a pineapple/ham, then a pepperoni slice.