The game mechanic is the bubble being invulnerable to harm and costing X amount of mana to cast. The actual ability itself like all abilities exist. Even the paladins in the Frozen throne and Warcraft 3 can use a divine shield I think its called.
edit: Even without the bubble, the paladin woud have to be a fool to take the hit, hes wearing heavy armour and a single arrow wound unless its a good head shot wouldnt stop him.