While crowns can save teeth, it does mean more effort from the patient in maintaining them. The crown circumscribes the tooth -- anywhere along the circumference where the crown meets the tooth (called the margin) is a possible entry point for decay. If the margin isn't sealed well or the patient doesn't/can't keep it clean, you risk getting a cavity under that crown.
Crowns are usually an investment of time and money from both the patient and the doctor. It is important to evaluate the prognosis of the tooth before putting a crown on it.