Metal accents compliment the design of your home and cover what logs or areas are left exposed to the sun, rain, and other conditions of your area. For example if there is a log that stretches out beyond your roof line it is usually best to either cut it off or to cover the top with metal. Snow will build up on this log and draw up the ends into the log. Sun will constantly beat on this piece of wood until it splits, checks, cracks then rots.
We also like to put metal on areas such as the tops of wood railings on decks since otherwise they can become difficult to maintain. If you have vertical logs on decks, or supports, and the tops are exposed these are akin to sponges for rain, snow and sun to build up on. Adding a metal cap prevents this direct exposure prolonging the life of your wood considerably.
In terms of aesthetics of a log home with metal accents, I personally feel that they look very sharp. It gives the home a finished, protected look - as if someone knows what they are doing. It is a subtle accent that can in many cases be designed to match your gutters. If you feel your home needs metal accents for protection, your options range from steel with a colored patina to copper or aluminum. Unless the house was very poorly designed there will only be minimal need for metal, and in some cases none is needed. That is, after all, why they are called accents.