Maybe they can't fix every problem, but overall, not bad to have around.
1. Find out during morning commute that your card is borked.
2. Go to the agent (if there is one) and ask for the form.
3. Buy a new MetroCard, same as your usual period since you can't be without it and the savings are worth it.
4. Fill out the form on the train, send it when you get off, and hope for the best.
5. If you're lucky, in a few weeks they'll send you a new MetroCard with the amount lost (based on the date your card was sent in until the end of its unlimited period)
Last I remember, you eat the cost of overlap since you don't stop getting an unlimited card in the meantime– though someone should correct me if I'm wrong here. I've actually never lost an unlimited to this problem though friends/family has.