Personally, and this is just my opinion, I see no need for more than 1 ball valve. I have 3 intake nipples on mine. 2 are just capped and 1 has a full port ball valve. If I want more air, I'll just uncap one nipple and fine tune with the valve.
Basically if 1 fully uncapped is not enough air, I'll just slighly open the valve. I'm just saying that with only 1 valve, there's still the ability for infinite adjustments from all closed to all open.
18.5", 22.5" OTS
Large BGE (chambersuac's)
The Fiery Furnace (chambersuac's)
Big JT's Smokin' BBQ Competition Team