Wet soap that isn't going to be used the next day, and stored in a dark place - can breed bacteria. Yes - bacteria can live and breed on/in soap.
Whether or not this will happen or be a problem is another story; one that disallows anyone to take a sure-footed stance on either side of the fence.
I would imagine that creams and soft soaps would be more suceptible to any possible problem due to their moisture content but I don't know that for a fact.
My basic feeliing is that wet soap that's rinsed off, then dried - then stored - should be fine indefinitely.
I usually shave with Williams - and that bowl is out all the time.. uncovered. The rest of my soaps are in a cabinet, covered. After using one of them, I rinse it off and dry it out for a day or so. Then it goes back into the cabinet. The exception is MDC - I snurdle a portion into a bowl when I use it, so it never gets soaked.