With American mortgage borrowers prepaying their loans at ever shrinking intervals, lenders might reasonably ask: Who needs such fickle customers? The unfortunate answer is that refinancing borrowers are just about the only ones around. Retaining and identifying the most profitable customers has become a key issue for mortgage lenders, said Geoffrey Oliver, managing director at BearingPoint Inc., during a recent mortgage servicing conference sponsored by the Institute for International Research in Chicago. And accomplishing