A circle is generally used for two purposes, at least in the spells and rituals I have seen.
1. As a protective barrier from Spiritual and Physical enemies.
2. As a summoning point for the dead.
A shield, however, usually entails the protection of Mental and Physical harm. So, combining wouldn't necessarily be bad idea, if the circumstances called for it, but don't go wasting energy every time you perform a ritual, casting both.