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
--firstname.lastname@example.org (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.