We can say "who cares" and "what's the big deal" all we want and believe it too, but the reality is that a lot of people are going to. As someone said, she's a serious actress who wants respect in the industry and a sex tape is going to hinder that tremendously.
I don't think she leaked it because her recognition and fame come from her own work and talent and it's obvious that her star is only rising. It was probably someone who held on to it until she was famous enough that he could sell it for a lot of money. The problem is that that's irrelevant.
I feel bad for the girl. I think in the heat of the moment we do things that maybe in hindsight might seem a bad idea.
Personally, I think that sex tapes are bad ideas, for women in particular, famous or not. You never know what is going to happen in a relationship and when it goes sour or when you're not with that person anymore there is nothing to prevent them from putting it out there. And people do put them out there. A lot of people.