Originally Posted by
BlazingPedals
Your experience clashes with mine. Try riding your bike for 50 miles wearing shower flip-flops, just for an extreme demonstration that stiffness matters. The longer you ride or the harder you push the pedals, the more it matters to have stiff-soled shoes. Note this is a separate issue from clipping in or using toe clips. Not having stiff-enough shoes causes hotspots and other foot pain. Of course, ill-fitting cycling shoes can do the same.
Now, for the person who rides on the MUP at 10 mph, cycling shoes are overkill.
Just because shower flip flops would cause problems doesn't tell you whether the marginal difference between riding with sneakers with a decently stiff soles and cycling specific shoes would make any meaningful difference in foot comfort. I ride hundreds of relatively intense miles in my hard-soled NB sneakers, and I've never had a hot spot or foot pain, and I'm 61 with bad feet and ankles.
Some fraction (maybe about a 1/4) of my riding is on MUPs, but I'm going a lot closer to 20 mph than 10.