Now that I'm a bit more awake, I can post another advantage that splitting your forces up could hold. It allows you to stretch out your BV to engage more of the enemy, though that only applies if you use the units from the previous waves.
As an example, let's say you have a 30,000 BV force going up against a 45,000 BV force. Let's go ahead and say that due to rolls, BVs in the actual combat scenarios are equal. Because of the difference in BV, you'll only ever be able to engage 2/3rds of the enemy force. However, if you do 3x Waves of 10k BV, you could stretch that out!
If we assume 25% of the remainder of the first two waves can move onto the next, the net result would be an effective BV of 35,625 (10,000 + 12,500 + 13,125). This means you'll engage 79% of the enemy force, as opposed to the previous 66%, and that number only increases if you have skilled pilots/tank crews who are able to have even higher numbers survive uncrippled.