It all depends on where you are going and what you are doing.

Many leisure travelers are going places where they'll be hiking, skiing, cycling, camping, diving/snorkeling, in cold temperatures. Many leisure travelers still want electronics - a small camera, an e-reader, a mapbook/guidebook. A pair of sunglasses in a case. 3-1-1 things that can't be easily bought at the destination (I couldn't get contact lens cleaner in Namibia - at all. An optometrist in Windhoek said he could get it on 'special order'). Depending where you are, it's good advice to take a small emergency kit - moleskin, head torch. You may want hiking boots and tevas on the same trip.

It's a bit hard to pack for two weeks hiking in Patagonia if you're trying to fit it all in a book bag.

I've packed in a book bag for a week in a big European city. I've also checked bags because things like trekking poles and hiking boots and down jackets or cycling shorts and a helmet either couldn't be easily bought at my destination or were too expensive to buy just to avoid checking a bag. I checked a single small bag last year. Two weeks cycling in the Netherlands and Belgium and I couldn't fit my helmet in my bookbag-sized carry-on.

There's no one-size-fits-all, 'my approach is the only smart way and everyone else is silly' solution.