I subscribe because I know that the author gets Karma if I do, and if I liked their story enough, I think they deserve the karma for writing it! Also, it makes their stats go up for having another subscriber. ^-^

I only subscribe to one-shots if the one-shot hasn't been written yet and there's only the foreword for it.
Otherwise, I don't subscribe. Whenever someone subscribers to one of my complete stories, I just go, "Why? Why would you subscribe to this?" XD

I subscribe oneshots so they are in my list and if I want to read the oneshot once again, I know where I can find it =)