Might I offer up that it's the same reason people pay $60+ for a t-shirt and $100+ for a pair of jeans that have a stupid label on them? When the material is worth less than a fraction of that? It's popular. I've hardly picked up a PHP book that didn't plug it.

It strikes me as redundant, personally. It's similar to my dislike of most frameworks, why use thirty pounds of product when one pound is what you need? I would say no, you don't need to learn Smarty unless you have an employer that orders you to.