i had said while the beta (on the french forum) that too much class had DoT and that will be a trouble on the endgame.
but well... it seems with ARR they have discover the joy of the DoT and place one (more on some class) in every class/jobs. it result in a futur huge trouble. actually we don't have a true debuffing class, summoner are DoT class, let's say they add thief as a debuffing dps, with limitation like this this class will be simply useless!

other point important, why put a limitation? memory trouble? (highly possible problem here) after all class like summoner need them dot for do all them damage. even the direct damage dealer like monk/dragoon need to put debuff/dot for be able to get all them dps.