I keep my guitars in hard cases, too, except when I go somewhere. They do a great job buffering indoor temp and humidity changes, and I can keep a humidipak in the case. You're certainly right that a gig bag I'd use costs about the same as a good case. The bag's just a lot lighter and more maneuverable. My well-constructed, modern gig bag is 9 pounds as opposed to 19 for my hard cases.
If I was still strong enough I'd carry a...wait...that's not true. Come to think of it, I'm not merely old and feeble. I also pamper myself way too much and I'm far too lazy to want to drag an extra ten pounds of dead weight like a ball and chain through an airport, or anywhere else at
any time in my life.
Though my admittedly expensive gig bag probably does as good a job buffering, I'd guess it's too permeable for the humidipak. So I store them in the wooden cases.
Perhaps next time I get a batch o' humidipaks I'll put one in the gig bag and see how long it lasts before drying out. If it lasts as long as it would in a hard case, about 3-4 months, I'm getting more of these gig bags!
