How does a cv joint break?

My 94 Grand Cherokee Limited is in the shop right now for a gas leak, and a broke CV axle. They estimated @ 400 for the CV axle if I dexcide to do it. The shop actually said if I am not 4x4ing, the Jeep would go another 10 years without fixing it, it will just make the noises, but otherwise is ok not to fix????