Normal everyday temp. fluctuations cause the always present moisture in the air to condense where an abrupt differential occurs which is the case where metal and wood interface. When you use and maintain the axe regularly then the fungus which breeds in conditions of condensation and moisture penetrated wood, will probably in the worst case cause a stain. That same axe stored up for a while will certainly suffer structurally. The problems with the pins, screws or nails, are similar but different but we all are familiar with pulling old nails and how a chunk of adjacent sound wood often adheres but the wood beyond that has rotted. S'why you get such big holes pulling old nails out. This is all actually not so central anyway, the point being to eliminate the need for such a guard by practiced skillful axe use.