But back on topic.... I have always been a fan of travelling light. This is possibly influenced by having down a bit of bushwalking/trekking early on where one has to carry all the time what one has take,. So it becomes a great incentive to minimise.
On my flight-travels I often move around a lot...and so for me it is not just the plane that is the incentive, but also the whole trip to have few and light bags.
I often use duffle type bags as you cannot beat their volume to weight ratio, and their should strap means that they are easily carried.
Many of my trips are to off the beaten track type places and so wheelie bags are often not useful..ie stairs, across sandy beaches, boarding ferries and boats etc..
There soft shape also means that they are accommodating to being squeezed in to spaces on trains, yachts, taxis and the like....which is very useful as my family travels are for 5 and so space is also often at a premium.
I also use Bags which are wheelie bags on some trips...but are still mainly soft-case and so their weight is still very light.