Personally, I've never had much of a desire to watch DVD's on my computer. To me, that's what a TV and DVD player is for.
However, getting a DVD drive is an affordable alternative for, say, college students that don't have a whole lot of space in their dorm room and not a lot of money to spend. That way, they can still rent DVD's to watch once in a while.
So, getting a DVD drive does have its merits. I just personally have no need for one.