mountian bikes on hiking trails

I run a cross country ski center in the White Mountains, when mountain
bikers first started appearing, we closed our trails to them. 
Mountain bikers ignore closed signs, so the battle began.  We were
convinced that they would cause erosion, litter, harass the hikers,
behave like snowboarders, etc.  I have come to learn that mountain
bikers are courteous, enviornmentaly conscientious, but have no money
to spare for trail fees.  I have also learned that mountain bikes
cause less damage than hikers, bike tires roll, hiking boots stomp. 
Tree roots receive far more damage from hiking than biking, because
hikers dig holes between the roots bikes roll over them.  Bikers do
more damage to water bars than hikers because they hit the same spot,
eventualy causing a hole to form quicker.  The local bike clubs send
volunteer crews to help on trail work, we have never had a hiker club
volunteer such services.