This bun fight has been well covered in the past, the problem being the forum software doesn't support any rating of usefulness as far as I'm aware so we are stuck with an artificial numerical value.
Even if it were possible as to who or what ranks a posts usefulness, if you base it on the forum users clicking a 'I found this post useful' button in the post. Then you fall into another trap, where some people don't bother clicking it at all and those who click everything.
Regardless of the ranking they are still clocking up number and unfortunately some people just see the number of posts and not that what they have said is of worth or not, but they think it is based on the number.
There are some very experienced and knowledgeable Newbies or Junior Members, as their forum ranking has no bearing on their personal experience/knowledge, yet their posts are very good, but they may be infrequent visitors to the forum.
So it isn't a simple matter to sort the wheat from the chaff.