You don't know you'll be facing a rifle round.
You could be facing friendly fire from CCW citizens trying to fight back. You could be facing shrapnel such as broken windows or ricochet.
It is costs verses capability...
The higher you go up in regards to protect, the price goes higher. You want lighter options, more costly than heavier ones (in the same level). You might not face a rifle round, but those Level III plates will stop handgun ammo just the same. Same with other projectiles... just not going to protect anymore than what the vest is covering. If you take a round in the head, a Level IV won't help. But for a shot in the chest, it will stop AP .30-06. My Level III plates are rated for M2 ball (DKX, for those interested).
You really don't want a hard armor plate, as your ricochet aspect comes into place. A round hitting a hard plate will spall (chunks of the bullet split apart and travel along the plate; if you run your hand from your chest upwards along your body, you have an idea where those pieces will end up heading if they go upwards). Personally, I don't trust some armor companies that take hard plate, put an "anti-spall" coating (pretty much, Rhino Liner), and act like it is safe. I've seen tests done where spall travels underneath the coating, stripping it from the plate, and continue out of it.
Now, I'm not saying everyone should carry armor, wearing it at all times. It is a good idea to have it for your home, for defensive situations. If you are issued it for work, definitely wear it. If you can purchase better armor (have the money without missing mortgage payments), I'd suggest that, as well.