MCI WorldCom fiber cut in White Plains, NY

It's all down to having enough spare capacity, and when you have half of
your capacity stuck in SONET/SDH protection, you cannot use that for
short peaks of traffic, and you dont have the option of saying, in case
of a outage, we will suffer 20% performance drop, which on expensive
circuits (like trans atlantic ones) could be what one wanted.

it's all down to the definition of "acceptable loss", then.