It could also mean that you're not supposed to use just one layer of defense...

I'm not sure who came up with this idea of 'layered defense', but it doesn't exist in poe aside from reddit comments and having support bitches. The only defense that mitigates all forms of damage is life. Period. All other 'defenses' require too high investment and provide too little actual coverage to justify the investment. This makes it completely impractical to use 2, let alone more, and expect them to sufficiently mitigate what they are intended to, let alone overlap enough to provide something you would consider 'full' converge.

This is whole reason why multiple supports exist.