Experienced warriors and soldiers can train armies, and I don't think those'll be in short stock in Hammerfell or Skyrim. The Legions may be well trained, but that doesn't mean they have to be in charge of the training of soldiers. The Jarls have their own soldiers, for one, which implicitly are not Legion, with the whole Balgruuf talking with his steward about accepting Legionnaires to bolster his own forces or not.