I am not sure why fire doesn't have that, but I would say that it is because fire has other strong spell effects and not all schools have everything. I think it's something like 3 effects per school.
As for defending, the Endurance attribute helps you with that, but I do not advise investing skill points in it, Intelligence and Willpower and more critical. Use items to enhance Marksmanship and Endurance.
EDIT: I have checked this out - it is stated that all schools except Healing have the ability to reduce marksmanship.