As it stands now..sure. Most of the guys I see running sups, would PREFER to be in a Plane.
Probably, there's some truth in that. However, hardening stuff doesn't help. I don't know what the average player is using to kill town buildings today, but it works nicely with 1klbs or 2klbs bombs (for the big splash radius). The heavy damage doesn't matter today, but it shows that hardening to anything below 2klbs is pretty much meaningless. Hardening beyond that would be somewhat ridiculous as Jabos couldn't even take down single buildings, and tanks would spend half their ammo on one building.
The problem with defending over resupplying is that these days its hard to catch an M3 with troops from slipping through. While GVs (especially unarmored ones) are easy to take down, it requires at least a certain degree of air superiority. During a capture attempt it is even at best, which allows both sides to slip through (with troops and supplies respectively). If the attacking side has air superiority (pretty common during a capture attempt), catching M3s from the air is pretty tough, but there's still a good chance to get through with supplies, as they can be dropped from half a mile out.
With a time from spawn to town of about 5 minutes or so - its not even possible to get there from another field, and absolutely impossible to get rid of the attackers (in the air) in time to create the situation needed to kill M3s.
(the only alternative is waiting in tower, to start defense early. and then only to take off to "defend" against light fighters that don't even intend to capture anything.)