Re. field use: in very cold weather, the metal can be cold for the hands.
Other than that, it's a bit of a wash. Rubber protects from dings and hits (but I don't typically hit the binoculars that much...). On the other hand, rubber, and especially the thin plastic over the hinge, can get damaged (by sun, oils, etc). Metal and leather are a bit more resistant in that respect. Leather can get scuffed if you walk through thorny bushes. Leather may absorb a bit of water and need to dry if you are in a downpour.