Fire mages have ridiculous ammounts of pressure and CC IF played very well.. BUT.. all of that goes out the window if a healer on there team has found his dispell button. Instant lb - dispell. Pyroblast. Dispell. combustion pretty much your only way to score a kill.. dispell.
All other DOT classes have some sort of prevention or protection for there dot's, from locks unstable affliction and the shadow priest talent Sin and Punishment.
All the dot classes rely hevaily on there dot's staying on the target. Surely it won't take much effort to have a specced ability like, "if your combustion is dispelled it deals 50% of its total damage instantly"