Syndra's harass might be safe enough to not have any defense but her ult range sure isn't. Mana regen is fine though, buy defense if you need it.
And scaling health seals are better for general defense at mid anyway.
Gold runes are better than any scaling stats, honestly, besides maybe mana regen. Your extra gold gain will progressively afford you better itemization at a similar rate as scaling health boosts HP, except if you decide you don't need health in the middle of the game you can just use your gold to buy a different stat.