Metal guard between brake rotor and wheel bearing assembly
 

I changed the front wheel bearings on my '01 Blazer and the driver side was missing the metal plate between the brake rotor and the hub assembly. I know it's not going to necessarily impede function, but what exactly is its purpose? Should I get it replaced ASAP or can I wait until next week when I have time again?

That's a "dust cover/shield" or "brake backing plate" depending on the manufacturer. It's supposed to keep debris out of your braking system. It's not a heat shield so you should be ok.

Mine rusted away and has been gone a couple of years now. No big deal imo!

It is an air deflector to direct air flow to the rotor which helps cool it.