The story that I have heard is that some providers have 1 (one) protect circuit to cover many primary circuits... You loose too many primary and limit number of protect circuits can't get them all. --asp@partan.com (Andrew Partan)

I'm not a TELCO wizard. The impression I get is that the protection circuit is intended to cover failures that take out a single circuit. For example, if you had 10 fibers in a system and a repeater or laser died, then only one of your fibers stops working. Fixing that with a protection circuit adds 1/10 to the system cost. (Maybe less if you have spare fibers in the ground.) Fixing it with SONET ring technology roughly doubles the system cost. A protection circuit doesn't help if your problem is backhoes.