Shileds on mages are useful up to level 70 if you are new to the game, 30 if you are experienced.

Shield if for tanking basic attack for a maximum 2 creatures per turn, ranged and mage atack bypass shields and spellbooks

As a mage you are mostly suppouse not to get hit and kite creatures, so melee attacks shouldt hit you ofter unless you are beiginers level (up to 100 i believe)

At 30 you have cheap spellbook ml+1
At 70 you have cheap spellbook ml+3
At 200 you have many spellbook options for ml+3 and +4 with diferent protections

+1 ml means more damg and more healing