The issue of encouraging combined arms by taking away collateral damaging 2-movement units (ie tanks) doesn't make sense to me.
One would assume the following scenario: Pre-Flight you could move a stack of 10 tanks into enemy territory, 4 promoted with Barrage, 4 with City Raider, and a couple of combat/drills for defense and here you have the makings of seige, city attack, and defense, all with one unit type! And meanwhile all those elite infantry upgraded throughout the years to CRIII, and successfully upgraded Accuracy/Barrage seige stand by and watch unable to catch up with the fearsome stack of tanks. With the advent of Flight, the Tanks job becomes easier and wars would be so speedy, the term 'war weariness' only applied for the AI's inablity to wage wars efficiently against each other. But where combined arms come to the fray in the age of the tank is when you need to hold what you've taken. Those ten damaged tanks can't successfully hold against a counterattack by themselves.
However I find in most of my warmongering games, the age of the tank is rather short lived (if you call a few hundred years of peace after the battering ram to regroup and confide in the economists, short lived) when I find suddenly all I need are a few paratroopers, lots of air/naval support and a lot of Gunships, and airlifted defense, to quickly wipe out an unsuspecting civilization or 2. I think combined arms is always relevant in any situation, barrage tanks or not.
My argument against taking away the barrage promotion for tanks is that the ten tanks scenario won't work, you will have to rely on the slow movers, war weariness does become an issue, and, well, its just not as much fun.