I've been using a hard foam and fabric lightweight case for flying for the past two years. I've flown about 50 times with it in two years and have had two instances of damage to my bass. That's a 4% rate of damage.
Flying to Colorado a year ago on Jet Blue it got a small rib crack. It was easily repaired. Coming home from Denmark last fall on Air France the neck was snapped off entirely. I actually watched this happen while changing planes in Paris. The bass fell about four feet from the top of a moving luggage trolly at a fairly high speed. The bass hit the tarmac, scroll end first and went end over end. There was no other damage, just the neck, which probably happened on impact with the pavement. It's hard to say if any case would have protected it from that! I know several bassists who have had similar damage in Gage and Stevenson trunks. The bass has been repaired and is sounding better than ever.
Since then I've stopped bringing my carved bass on the road and bring an Upton Hawkes Laminate instead. It's a great bass and much more durable to the vigors of the road. I wouldn't recommend flying with any bass in any type of trunk that is not heavily insured and replaceable.
Personally I think you are about as likely to sustain damage in a lightweight trunk as you are in a heavier one. You are really taking your chances anytime you put an instrument under an airplane, no matter what case you use. I personally like my 4% odds better than I like stick basses, renting, loaners or being refused at the ticket counter in an airport.
There are two significant advantages to the lightweight case that I can see. One is that you are never overweight. My case with the bass in it weighs 49 pounds so I always get it on the plane. I don't even get charged oversized about a third of the time, even though it clearly is. With many airlines dropping their weight restrictions down to 70 pounds for overweight, this ensures that it gets on the plane.
The second thing is that because of it's light weight, I think it gets abused less by baggage handlers. It's so much easier to deal with and move around. One person can easily lift it. It's got a good set of wheels and is really easy to manage. Also because of the light weight, if it does get dropped the impact is a lot less and the foam absorbs the shock more than a rigid fiberglass material.
The disadvantage is that it is not nearly as durable as the more traditional trunks and gets torn up pretty good. It's strong enough for me to stand on it over the bridge without it caving in at all, but I'd imagine if it were crushed under something really heavy it would eventually give. The likelihood of it being under two hundred plus pounds of sustained pressure is anyone's guess. Again, I think you take your chances no matter what and so far I've had a decent experience with it.
I know at least two other bassists who are also using this type of case to fly and as far as I know they've had no damage to their instruments.
Hope that helps.
Corey DiMario
www.crookedstill.com
www.lissafiddle.com