The purpose of a temporary is to protect the underlying vital tooth (if vital) from sensitivity and prevent the teeth around the prepared tooth/teeth from moving. The temporary must be strong enough to resist breakage for a short period while flexible enough to be easily removed.
I do many crowns and place many temporary crowns and if the permanent crown is to be placed in the next few weeks, my temporary crowns almost never come off or break. This is when they are cemented with the weakest cement. Sometimes I have temporary crowns on for a number of months for various reasons. I can still say that we have a fairly high success rate but you must remember that they will not last more than a few months because they will loosen as the temporary cement leaches out and the tooth is subjected to the forces of chewing.
All the best
Dr. Ron Barzilay