I jump on the bike in what I am wearing, except never sandals for flip flops.
I always ride in bike shorts. I just find them more comfortable. But I'll wear sandals and a tee shirt for shorter rides. Birks have a nice stiff sole that works pretty well. I've been contemplating trying them on a longer ride as they are much cooler in the Florida summer.
When I was young, we'd ride a century riding commando in Levis. People should wear whatever they find comfortable. Who cares what anyone else wears?