It depends on the mid. Flat MR and Flat Armor really don't do a whole lot (We're talking, effectively, between a 7 and 10% HP increase against that type of damage). So, 13 MR is going to be 70ish HP increase for most characters.
So, if you're constantly at the mercy of ranged attacks (Such as Akali or Fizz), that 70ish HP would matter. If you're highly aggressive early and all in (Yas, for instance), then it matters. If you have a way of regen/sustain, (such as Vlad, though he probably shouldn't be mid) then it matters. But if you're ranged and aren't at constant mercy of harass like Morgana, Lux, etc? It's not needed.
Most mages are better off with scaling CDR simply because it allows them to max CDR with just morello's and masteries. That will give them way more survivability than an extra 70-200 HP.