Polycarbonate crowns are primarily used as a temporary restorative measure because they provide adequate esthetics and protection for a short duration but lack the strength and durability required for permanent full coverage. They are easy to fabricate and remove, making them ideal for provisional restorations during the interim period before a definitive crown is placed. Their material properties do not withstand long-term occlusal forces, hence they are not suitable for permanent restorations. Additionally, their translucency and shade match allow for acceptable esthetics temporarily in anterior teeth.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition.