The problem is if you changed Shield this way it would become too useless at lower levels. At low levels, most attacks are going to be for 1 damage. With your proposed solution, Shield doesn't help at all.
Would you also suggest the same changes to Void?
It would be interesting to see actual usage statistics.
I see the reason for it working like it does, but there are so many cards that are themselves worthless. Most of the cards with shield are almost worth their mana without shield, whereas many other cards are too expensive without having to fight shield, and when against shield, they are moot.
I disagree.
Frozen Soldier, Tortisian Fighter, Naga Fire Wizard, and Skeletal Warrior are not very strong cards even with Shield. (Their main weakness being very vulnerable to magic damage).
Serpentine Soldier (at lower levels), Frost Giant, and Gloridax Guardian have narrow uses but generally aren't that amazing despite having Shield.
Elven Defender and Chain Golem are rarely used in Ranked and All Edition tournaments but are actually very strong in Untamed only events.