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.