When to choose tabular models over PowerPivot models

Sometimes people ask me for the slide deck to my PASS presentation. I send out the deck, but there is only one slide in there that is worth anything. This post summarizes the only useful slide in the entire deck.

Let's think about this question - why are tabular models necessary when we already have PowerPivot? After all, tabular models and PowerPivot models use the same engine and have the same capabilities for defining rich business logic in DAX. When is it necessary to migrate to tabular models?