Nami Tidecaller's Blessing (E) mechanic

Hi, I address this to Rito mainly but do not hesitate to give your feedback.

I like to play Nami with a lot of Ap to have big heals, big ms boosts, and big slows while having some damage. At the end of the game I have my sorceress shoes with a void staff plus mr pen runes but I see on the wikia that Tidecaller's Blessing apply the magic pen of the owner of the passive.

That mean for the ADC that I support, 0 mpen.

I wonder if this spell could use the bigger magic pen value between Nami and the guy with the passive. I think that would make sense.