I surely hope there was something concrete behind that advice. Otherwise it would be like telling Uchiha not to use Sharingan or Hyyuga not to use Byakugan. Or Gaara not to use sand. Of course it makes sense to not use it all the time, because unlimited pool would mean little need for control and that would mean lessening professionality, but if he's going to go against Akatsuki dudes, it surely should be a minor detail how cleanly he fights.
Besides, whatever dangers may be hidden in using the sealed powers, it should be obvious that the better he is able to master and control them, the less chance of any fall to the dark side there should be. But if he never uses them, then it would be like giving an assault rifle to a chimpanzee.