Mulceber, this is a topic that has received alot of attention in historical posts...I suggest you search the forums to get the full body of information. For example, check out this thread.

Yes, command stars do have a positive effect on the outcome of battles...especially in auto-calc. But when you're in control, you're much better off having a general that gives lots of morale bonuses (which boost the performance of his entire army) instead of one with lots of command stars (which boosts the performance of a subset of his army that is in closer proximity to him).