If their Rumble laneswaps with Zed and leaves you stuck with two Ruby Crystals (!), would it not be better for you to just laneswap as well? In the worst-case scenario, is it really that bad if you end up with health? Had you invested in a Null-Magic Mantle, you would've been completely screwed, but at least the Ruby Crystals allow you to defend yourself a little bit better, which is a significant advantage. Buying health means that, no matter what, you're going to be a little bit tougher to kill, so even if your opponent laneswaps you still have an immediate measure of defense as you build towards a new item (armor is also not that difficult to start building, unlike magic resistance).
Put another way: you're laning top against
, and you get a lead on her while you build
. After shoving her out of lane, you decide to gank
at mid, whereupon she unleashes her full burst rotation on you. Had you built armor, you would've had no serious defense against her, but with a Giant's Belt you get to absorb some of her damage, while also protecting yourself against Irelia's physical (and true) damage. Unlike resistances, health protects you universally, so if you get tanky on health, you're tanky against pretty much anyone, and not just physical/magic damage dealers. A few seasons ago, health was extremely easy to access, and the result was that most top laners got very bloated very quickly and got too difficult to kill. There are a lot more ways to deal with health stacking now, but it's not a stat you want anyone to build up too quickly (or too easily).