PNC, page-41

At the end of the day when the debt is either collected or written off the only difference between the two methods is timing. There's nothing to say that if the economy changes, fair value won't be lower than cost.
The key metrics are the age of the debt & whether it's being written off.
Historically PNC have collected more than they've paid for debt & therefore made a profit.
PNC still have to provide for debt that is not likely to be collected irrespective of whether cost or fair value is used, so 'cost' is not really what it's cost to purchase an individual it's actually adjusted to what you could call 'fair value'!