As stated in numerous replies, it depends on the investment involved, and that should be something that common sense would dictate. Currently, I don't own anything that would be gigged without cases. I don't think there is a type of gig I'd be interested in that would allow a non-cased kit, i.e. a beater kit for a Blues jam, etc. But I could see myself getting interested in one of those late60's/early 70's Slingerland concert tom kits - as long as they are the 3 ply maple with top and bottom re-rings - to be available for the odd week-night jam hosting activity. Those sets are still around in the $400 range (typically sans snare) for a 3 piece kit and you could nest a 12/16/22 into one case OR just throw them into the back seat of a car without cases. I had a friend that used this setup on certain gigs, and he was a fairly prominent name. I laughed the first time I saw him take the kick with the 12 inch tom still mounted, then the floor tom with legs on it in 1 hand, snare in case in the other, lightweight stands in what looked like a rifle bag/case, and cymbals in the other. He was in and set up in less than 15 minutes. If I buy another set, it will be modeled after this.
Back in the 60s when I was starting out, I don't remember any of the drummers in my small orbit having cases for their drums. Even my teacher, a weekend warrior, didn't have cases. We all just hauled our drums au naturel, and I don't remember anyone having a drum damaged in transit. Car backseats are padded, after all.