Thank you for the definitive answer legoburner.
To answer your question SilverMK2: I found that it helps me understand the users who frequently "lurk" a certain thread. I understand many users use many different sub-forums, but at the same time I imagine there are those like me who only really call two or three (maybe even one) "home". It is nice to see who the users are that stay in one forum and thus see the widest range of content in said forum. I find this particularly useful in the background and fan fiction forums.
For background, it shows who really is a... well a fluff expert. Someone who always sits in the fluff forums can generally be viewed as a fluff enthusiast and thus it can be assumed that they know more than someone who comes in, reads a thread, comments, and leaves.
I don't mean to impart a feeling that this tool adds to sub-forum focused "elitism". I have never experienced that, though I suppose it could be a side effect.
For fan fiction the same can be said.
That is to say they enjoy reading fan-made pieces and it can be assumed they do more of it than most. Someone who hangs out there a great bunch of time ("great" being an adjusted term depending on how active a thread is) can logically be said to see a wider range of content and thus their comments and critiques are more valuable. It is important however, to understand that this is not a rule, but just a general truth. As such there would be many exceptions.
Which is what this feature really boils down to. It shows you is taking their comments most seriously when in their "home" thread.
Let me say that it in no way diminishes the value of other user's comments; it just lets you see that "User X" hangs out here a lot, seems to know what he is talking about, and thus can be generally more reliable and constant in opinion and/or facts.
The same result can be had with long stays in said forums where the feel of it and it's users will eventually become known over time. This little add is just a helper.
But in the end legoburner brings up a good point, and one that overrides an above-mentioned positives, so I'm in no way sore on hearing a "no".
*edited for spelling and content. edited content in red (2nd edit and 3rd

).