The truth of it all is in the link provided earlier. Even with a .6 modifier, an add that pops in the middle of a fight can be very difficult to control. If you as a healer happen to pop a medica2 and crit for 1500+ on an entire party or raid it is quite possible that a tank cannot even generate enough enmity within the mobs lifetime to get it back. An overzealous dps can easily end up tanking with a few lucky crits or if they jump on a mob as soon as it spawns as it takes a tank 7.5 seconds (possibly less if they have tons of skill speed gear) to do a full combo rotation, which usually works out to be only slightly more threat than a single rotation of a good dps.