The list reveals an interesting pattern of groups being ignored.

1) incoherent posters
2) relentless self-promoters
3) ponzi scheme/fraud whistleblowers

I noticed my button get much darker after posting about the scams and scam-enablers (including forum admins). This got much more obvious after I called out Theymos for his role in the matter and his admitted and knowing participation in Pirate's ponzi scheme. If scammer scumbags are using the ignore feature to label those who oppose their scheming, then I leave it to others to decide just how useful the feature is.