What exactly is the 5-star thread rating system for? Are there known random trolls who just rate a thread 1 star because their day was shit?

Yes.

many knowing not that rating exist, so used not many.

"Are there known random trolls who just rate a thread 1 star because their day was shit?"

Appears so, I have noticed someone loves to give 1 star for about all threads... and I thought I have too much time..

I suspect these are either disgruntled former members (like "Tapoktro") or people in the industry who want to discourage us from posting stuff.