Look, pineapple is awesome and if you don't want as much acidity (like my wife, who has troubles with it) check out white pineapple instead.
All About White Kaua'i Pineapple – FruitStand.com
Now, on to pineapple on pizza. Yes, in certain circumstances. Straight up Hawaiian is definitely OK, but throw on regular bacon and/or pepperoni instead of (or with) the ham and it really starts to shine as a combination.
Those of you advocating as burger purists are missing one of the best things about burgers - dressing them up different ways! Like the Captain said the Western or BBQ burger (typically bacon, BBQ sauce, cheese, and onion ring or onion strings) is a fantastic classic. Hawaiian burgers with a pineapple ring and teriyaki sauce are great as well. Also good is throwing on some pastrami and swiss cheese. Heck, go check out the combinations at places like Stoopid Burger, or Yaks on the 5, or Squeeze Inn (cheese skirts!), etc. Lots of good combinations!
This is the biggest reason why In-N-Out is way overrated as a burger joint - not enough variety. They make a "fresh" version of the Big Mac, essentially, with worse fries, and they have hoodwinked a ton of folks into thinking they are great. If you don't like their idea of a burger, then you get nothing. Meh. They are fresher and aren't expensive, but they aren't great by any means. Yes, I said it.