plugs and caps are used for 2 reasons. 1) they are cheaper than ball valves. 2) by removing caps, you get pretty much complete control of air from 0-3 open.
There is no real reason for the added cost of 3 ball valves when 1 ball valve and 2 caps can do the same thing. Depending on your desired temp, you will often start all open, then usually cook with just the valve (both caps closed). If you have damp charcoal or want more heat, you can always remove a cap and get 1+ the valve open.
While I never had a huge problem with an uneven burn using 2 caps and a valve - if you want to go with one huge intake, you might want a deflector. w/o a deflector my guru did have an uneven burn.