Most Americans use a fairly sweet pizza sauce. Adding pineapple to that makes it too sweet. But I like my sauce with more tomato acid, so a little bit of fig or pineapple complements the flavours. The nice thing about pineapple is that you don't get it in every bite like you do with a sweet sauce.
Edit: and now I sound like the pizza snob. :) It should be 7 -- good pizza has both crisp and chew.