You can cut up your pizza and eat it like cereal out of a bowl of grape juice if you want. And if all you're after is fruity sweetness you'll probably even like it.
But there are so many things that go well on pizza (pepperoni, salami, bacon, hot peppers, mushrooms, olives, sausage, artichoke, asparagus, fresh corn, and on and on) that it is a tragedy to spoil it by drowning it in fruit juice, BBQ sauce, or Thousand Island dressing.
These are things that change the entire nature of the food; they are not compatible with pizza. It's a fruit product with pizza-like extras. You're certainly not going to taste anything but soggy pineapple juice bread.
I mourn not the grossness of the pineapple pizza, but for the pizza that will now never exist.