Agree with Kraxis about unit stack limitations and usefulness, except for one thing: there's so few of them that the 'experience' rank tends to be easier to improve and thus eventually create a force you CAN justify taking up a stack in your army.

I train them up by using them exclusively against rebels. It's a bonus that 'retraining' keeps the experience level, but without it i'd simply do more micromanagement anyways, so it's nice to have this convenience (especially with 9 ranks instead of the old shogun days)