When to use curbside:
If you are non-elite and the regular check-in line is long.
If you have lots of heavy bags and no skycaps are around.

When NOT to use curbside:
If you are seeking an upgrade.
If you are changing your itinerary.

Curbside check-in is often handled by glorified skycaps, who may or may not have a firm grasp of the nuances of your particular travel habits and/or needs. In my experience, they have always been able to check bags, assign seats and produce boarding passes. Also, curbside check-people will expect a tip of either $1 or $2 per bag.