On one hand, yes you can recruit people and tell them to shoot at it until they're great marksmen. On the other hand, time spend recruited is time that they are not working, eating, or sleeping. For the entire time that they're training they are producing nothing for the colony, and if left for too long they'll have a mental breakdown.
In other words, keeping soldiers in perpetual training becomes a drain on your working population. Just like real life. I think it balances itself out.