What Grif is saying is that the two types of metals would mechanically work the same, just with different empowering and disempowering elements.
And as I write that, a rebuttal comes to mind for the existence of both types of metal; if the dark version of Shimmerite can be purified/negated/de-powered by light magic, then why can't the light version of Shimmerite be corrupted/negated/de-powered by dark magic? If they really are the same metal with flipped elemental properties, then they should work the same, right?
So, if your grand magical paladin power armor - your ace in the hole, the thing that will give you the edge over the villain - can be rendered moot by a school of spells that are almost exclusively used by said villains?
Seems like a bit of a major design flaw to me.
The changes Grif is proposing fix this problem. Dramatically, IMO. The armor grows stronger and harder the more it is exposed to dark magics - things that paladins will run into quite a lot - and can be cleansed with a good dose of light magics - things that paladins will most likely have. When in the right hooves, the armor turns into a dual-edged sword; evil's strength is turned against it, while the paladin must constantly keep his armor in check, lest the darkness within it grow too great. The very model of self-sacrifice that many paladins seek to embody.
When in the wrong hooves, it turns into a selfish mockery of its intended purpose; a self-sustaining engine of destruction that corrupts the already black heart within it. But as the user trades long-term morality, wellness, and possible sanity for short-term power, there is a great cost. One Good mage, one mighty holder of the Light, and what was their source of strength becomes their prison and soon-to-be tomb.
That, to me, is far more interesting than a metal that essentially plays Rock-Paper-Scissors with alignments.