If you don't mind another opinion............When clicking into a thread, it always takes me to the end of it anyway, which is where the Reply Box is. I don't have to go there myself, it's automatic. Isn't that what it should do?
Or do you mean you'd like to add a reply to a specific comment, somewhere before the end of the thread? I have that option on another site I visit, and I don't especially care for it. It seems to get jumbled up, putting a comment in the middle of existing comments.
I'm curious to see what Dave will say about it.