Ask yourself,
Validation only occurs when the # of confirmations increases,
Confirmations only increase when a New Block is added,
Therefore non-mining node don't validate anything ,and are irrelevant to network security.
Non-Mining nodes could all turn off right now, and no one would care.

I will tell you a mind blowing truth:
Miners do not validate transactions.
Full nodes do. Anyone can validate transactions, a mining rig is not necessary for that.