Giltia, your champions need AP to farm. Mid- to lategame their base stats just won't work, and they won't have AP if shut down early.
But since Singed's poison isn't limited to cooldowns, Karthus and Anivia are likewise excellent farmers, as GoldDraco pointed out. But they aren't played on 3v3 (which has its own ladder; it merits some balancing).
Singed's role, whatever one calls it, invloves staying alive to some degree. With his 25 second ultimate and effortless farming, he's as good as any tank in this respect.
The difference? Other tanky champions, if thrashed earlygame, can't farm well, which is the crux of my argument. To clarify: Singed farms amazingly well even if he has a horrible early game, allowing him to too easily fill his role(s) every match. Agree or disagree, I hope you at least understand what I'm saying.