When I punch/kick in the thin air, or do my solo throwing drills, I just feel like I'm masturbating myself. There is always an empty and lonely feeling in my heart. When I punch on a heavy bag or throw my throwing dummy, I can feel that I'm doing something meaningful.
You should put some free form jazz music on in the gym next time and you wont feel so lonely!
Anyway - i think whats been missed in a lot of the criticism of 'air punching' is that a lot of CIMA form really relates to rou shou type practice. Solo form is a compendium of technique - memory tool, but it truly makes sense when you contact somebody (and struggle a bit). One can also refine spinal wave etc, crossing the body for power generation in the soft solo way with good results. E.G: I got a pretty good low kick developed from Gao bagua circle walking but i did do pad work too. The difference of adding the solo form I see when ever i do kicking practice with pads with Thai Boxer types etc i seem to get some approving looks... its my TKO move I kick waaay through the pad/leg.
What John says about retreating under pressure with good structure is so true. I think shadow boxing can help a bit here - but perhaps better still get a pad holder to chase your backwards as you defend.
In my experience structure building is a slow experience. Relaxation is key to it - so solo 'air punching' is supplemental way to drill the co-ordination with relaxation. I do think bag work is very important short cut to compounding the structure. To cut it out of training is like cutting off your nose to spite your face.
P.S Mike Sigman uses a heavy bag in some of his videos. He's pretty good in my opinion so maybe he got something from it.