Or is it just the fact that it's essentially nutrient-free energy?

When they say you shouldn't eat it, is it because they are assuming everyone is a fat fuck who doesn't need the extra calories, or is there something specific in sugar that damages you?

Put another way: I'm tall, skinny, probably a little underweight. I have one of those metabolisms that just burns through whatever you throw at it. Is there any reason for me to drink sugar-free coke instead of the real thing?
>inb4 yes it tastes better
No it fucking doesn't.

Also, if sugar really is bad for you, does that also apply to the sugar in, say, fruit or milk?