My dress shoe collection is probably 70% leather-soled, the rest split between rubber (Vibram) and Dainite. Do I think leather is more "formal"? Absolutely. But I they're all appropriate with suiting, I'd go as far as to suggest that if I chose to wear the leather-soled shoes in crap weather out to a client meeting that someone who knew I had other better options might think less of me for the choice.