Why was the AP removed from Banner of Command?

Because it was strong on Zyra and Heimerdinger when it used to passively buff their minions.

Lane pushing items are usually built when behind. Tank stats are good when behind. It's actually pretty useful now on losing teams when you want to push out a side line while you clear out super minions in another.

I feel like banner's effect is very limited even for defending. It's like your hoping enemy team doesn't notice the buffed up minion and doesn't try to push out lanes that they haven't won yet. At least with Zz'Rot those creatures are summoned continuously.