I admit, sometimes when I'm doing a heroic I often forget to switch from Crusader Aura to something more useful.
There are a lot of players who are so happy to remind me I've made a mistake. I don't know why a hunter would care that we have Crusader instead of Concentration, but evidently he does.
With the sick sense of humor I have, I often just turn it off. So when the mage sees the Crusader icon disappear off his buff bar, he thanks me. He didn't realize that instead of a useless aura, he no has no aura at all. And I get thanked for it!
If you go aura-less throughout heroics, few if any people will ever even notice (I forget to put an aura back on after switching specs, most often). Yet if you leave up Crusaders, you will surely hear about it!