What I learnt is that you have to get used to it. I can wear them now, because I just got good at it. I do love my birkinstocks for winter, and I'm going to get myself some sandal / thong ones for summer soon.
I have learned that after the Winter break in closed in shoes/boots, it takes awhile for your toes/feet to adjust to sandals and be bit between the toes. They must toughen up a bit cause after a little while all is sweet and....I can tiptoe thru the tulips!!!!?
I've never worn thongs/sandals with that bit in them as I think that bit (of skin) between the toes would be so sensitive.
I've seen people on the train complaining after a day of walking around town wearing them.
Plenty of other style sandals without the 'bit' to choose from.
I used to love wearing things, but now, as I'm getting older, they seem to hurt between my toes, & I think I'm to old for them anyway. I now only wear Sketchers every day when we got out, or short boots with thick black cotton sox & marino leggings during winter.