User blog comment:Khaladar/Rasquerion's Heartswapper/@comment-4834593-20120617204925/@comment-5159141-20120730211950

Simply, to have a choice. You might want to build only Armor/MR, and benefit from that particular regeneration bonus this item grants, to synergize better with your build. It's also to regulate the maximum strenght of the item, as you can build great amounts of Magic Resist or Armor alone - but to have both stats, you would need to equalize the amout between the two, thus reducing the individual effectiveness of those bonuses.

That said, Rammus still would be a beast with this item.