Under conditions of processor Vcc going down to ground and then coming back up soon there after in a blackout type situation such as could be caused by a user dropping a battery operated product on the floor (the spring contact opens for a time) can lead to latchup. What happens is somewhat the opposite of what you expect. The low bias fed to an I/O pin that is avove the VCC pin by a significant amount can arm an onchip stray PNPN or NPNP junction. Then when the large current flow returns to the Vcc pin upon power recovery there is a period of time where catastrophic latchup can occur.

I'll keep the diode. I use BAT54C types for these anyway. You get two diodes with common cathodes in one SOT-23 package. So you can protect two inputs with one component. In large production quantities the BAT54-C is less than $0.20 so each protected input is costing a dime.

Michael Karas