... is a time-tested Slashdot commenting strategy!
But seriously, I don't always carry my wallet with me, but I almost always carry my phone with me. Last year I found myself in the perfect position to benefit tremendously from a mobile wallet on my phone.
I was on mile 4 of a long bike ride when my rear tire failed. Not the tube (I carry a spare), the actual tire. I had decided not to bring my wallet with me, but I did have my phone. Anyway, I needed a replacement tire, but I had no money on me, and I realized that despite having my credit card number memorized, I didn't actually have any direct way to pay a bicycle shop for a tire, so I walked home.
But it felt silly - that I was carrying around a smartphone that has access to multiple bank accounts and payment services, and that I even knew my credit card number, yet without a little piece of plastic, I couldn't pay for anything.
Since then I don't go on bike rides without my wallet, but that's not really the point. Sometimes I take walks and don't want to bring my wallet. Occasionally I change my mind on the way home and decide it would be a good idea to stop at the grocery store. But no wallet, no way to purchase anything, despite having my phone.
In other words, there do exist situations in which one might reasonably have a phone but not a wallet. You may argue they are edge cases, but I am just one person. Other people mentioned check splitting, which is especially a headache in recent years since no one seems to carry cash anymore.