Singed is one of the few tanks that you can legitimately afford to just "ignore". If you're not being directly led into his miasma stream (by chasing him), it's awkward to place around a team, so it's not likely to hit more than even a single target in a team fight. Not to mention that it's magic damage (and not even that strong), so it can be easily taken care of just by buying a Negatron Cloak. The only REAL threat the Singed poses is his fling, but that is on a rather large cooldown so in a team fight he only gets to use it once, and usually your carries are hidden behind a tank or two who can afford (if not enjoy) getting flinged into the enemies. Singed's real threat comes in that you THINK you can kill him, because you can usually burst him down to like 35-40%, but his health regen keeps him through your follow up sustained damage. There's a lot to be said about the psychological -want- to kill someone who's low health... it can fuck up a team's ability to focus the correct target. It's the Mundo effect. If you just realize that you're NEVER going to kill the singed, he becomes much less threatening.
So in summation: don't chase the Singed.