The other obvious reason for T2 being effectively skipped is cost-effectiveness. I've put a load of figures into a spreadsheet and come up with the following:
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I have ignored the Aeon Swift Wind Combat Fighter and the Aeon Restorer Gunship in the above figures, because they are substantially different from the other factions' equivalent unit.
Please note that the damage figures are DPS for fighters and gunships, and damage per drop for bombers.
What this seems to show is that T1 and T3 interceptors have effectively the same damage and hitpoints per mass. The energy costs are quite different but this just lends weight to the argument that it is mass rather than energy which is the limiting factor, even though air is ostensibly limited by energy.
The T2 fighter-bombers cannot compete with the T1 & T3 fighters at AA; it costs over three times as much mass to get the same anti-air firepower. Their ground damage is more impressive, but it is still lower than the equivalent in T1 bombers. Strat bombers are worse on paper, but in reality their role as front-loaded heavy hitters and their extra durability and speed help make up for their lower damage per mass.
As for hitpoints, the T1 & T2 bombers have roughly the same amount per mass cost, the T2 edges it slightly but it's only a matter of 6% difference. The weakness that both T1 and T2 have is flak: a small amount of flak can decimate any T1/T2 air force extremely quickly. Again the strat bomber makes up for its comparatively lower health by being that much harder to shoot down.
Should anything be considered to bring T2 air more into the game? What could be changed to achieve this?
If anyone wants to see the full list of figures, you can download the spreadsheet here.