I have to outright tell my static healer that if you're knocking something back, I'm not going to tank it (she gets to learn the hard way). The way I pull keeps track of everythings positions. I can't have something suddenly hitting me from a side once it wanders on back. The tank is generally going to be the one keeping positions proper for DPS to do their positional attacks. The tank doesn't want someone else taking control suddenly. It's like someone pushing you out of the drivers seat in a car.
That being said, I'm currently levelling WHM and if I encounter tanks that like to number targets, I'll usually pick the highest number to send flying off so he doesn't get hit by it for a while. This is usually only the case in levelling roulette where a low level tank may not be adequately equipped making him harder to heal.

So I'm a bit of hypocrite, but if I'm ever asked to stop, I'll stop. I don't want to stop the tanks 'flow' because I know how annoying it can be.