Hmm... that's a fascinating idea!
I used to scoff at most memberships (both from the side of value-to-me and value-to-companies), but... now I not only love my Costco and Amazon.com Prime memberships, I also patronize those stores WAAAAAAAY more than I would otherwise.
With Amazon Prime, it's almost become comical. Need more toothpaste? Amazon. Night lite? Amazon. Mother's Day gift? Amazon.
Heck, call it United Prime

. I'd be really surprised if they couldn't figure out a way (much like they've done for those of us with status) to essentially prevent people from exploring alternatives when booking travel, even when other options might be slightly less espensive.
In a nutshell, then, it seems to me that memberships would enable airlines to get past the hump of insane (and hugely damaging) consumer-uber-price-sensitivity (ohhh, this carrier is $3 cheaper...!). Fewer price wars, better service... ah, I can dream, can't I?