Honestly, it changes the metagame more than anything else.
It might mildly change deck-building in tournaments with strange or specific rules.
I honestly think that is the biggest effect.
Well, than an it might mess up some AI, who might play this on top of another shield. (AI tend to replace shields, just because they can, and they usually will be able to afford this)
EDIT:
I realized that this post didn't make my opinion clear enough.
I completely and totally agree with Essence in this case.
I often respect a thematic argument, but in this case, it is ridiculous.
In terms of it being balanced, OldTrees says it is balanced, and, before I read that, I had already come to the same conclusion.
He did a good job explaining, and, though I did a slightly different calculation, I don't feel a need to also post my work here.
Now, people say it won't be used. I think it will, to some extent.
You don't see people looking at dirk and saying "This is ruining the game! It is so unlike
to have an edged weapon!"
This is the same type of argument that Valimont is making.
Thematically it actually works with the game: it makes sense that if there can be weapons that are not aligned with one specific element, then of course someone could slap some metal together to make a shield.
Essence, + Karma.