So I've been playing Jinx on Twisted Treeline a lot lately...
And it's really made me realize what few options there are for magic resist on ADC's after the mid-season update. Not only does GA no longer give magic resist (though GA isn't even purchasable on TT so even if I just wanted to buy it for its passive I couldn't), but Banshee's is now an AP item as opposed to a generic AP. I wish their were more options when looking at what to purchase in the store, and I am beginning to feel like mid-season was a huge step back in this regard. Instead of increasing item diversity, it locked certain items into certain classes.
. Although it does suck that there isn't an option for both armor and magic resist, like you said.