To be honest, though it isn't very nice, it doesn't cause them too many problems and the mud breaks out every now and then. It certainly doesn't cause their feet to grow wrong. If you watch them walk, on certain types of ground, the two parts of their feet splay out quite far naturally.
What can happen though is that the skin between the two part of the hoof gets soft and then attacked by a bacteria causing scald. This is sore and they'll limp. It just needs spraying with Terramycin foot spray.
They will thank you though, for whatever you can do to provide a dry path. Some folk use bark chippings - temporary, some use that carpark plastic grid and put stone chips in it, some use paving slabs.