I have heard many times that putting you best unit wether it be in CC or Shooting against you opponents best unit is not the best idea, and that your best unit should focus on the weaker units?
Yes ork boyz are a weak unit but even that depends on size and circumstance. For instance a unit of ten isn't going to do anything but a unit of thirty can shread terminators and tanks. However, if they don't get the charge then they're quite easy to kill (speaking from experiance).Ork Boyz a weak unit?
They are one of the best units in the Codex. In fact, they are one of the best units in all of 40k.