The rapid-fire bank merger market of 2006 and 2007 has died down considerably since 2008, but most consider it a resting period before the cycle picks up again. When that happens, banks will have to deal with a familiar problem – hanging onto customers at the gobbled-up institution.

If, that is, they think those customers are worth it.

Recent research from Deloitte Consulting, which was presented at the BAI conference in Boston last month, shows about 15 percent of customers at the acquired bank will head for the exits, post-merger.

And these are often high-end customers, says Thomas “Toby” Kilgore, principal with Deloitte; it’s a general rule that the more products people have, the likelier they are to switch.

That can be a damaging exodus, he said, especially since many banks expect that enlarging one’s market share by acquiring a bank in a new area will yield all or most of those bank’s customers.

If the acquirer doesn’t front-load their retention efforts by understanding those customers and communicating with them early on, they’ll be lost. It’s worth noting, he said, that the largest chunk of customer leaves early; about 85 percent of the people who leave do so within the first three months after a merger.

Richard Collins, president and CEO of United Bank, said he and newly acquired Commonwealth National Bank communicated early and often with Commonwealth’s customers, sending three letters before the merger was even finalized at the end of November.

Commonwealth CEO Charles Valade said customers are most worried about the bank employees they work with, a statement echoed by analysts at BAI. Because Commonwealth is keeping its personnel in place, he doesn’t anticipate much disruption.

Bank analysts agree it’s vital to have a game-plan for hanging onto customers, and that a loss of core deposits is a bad thing. But, says Eric Luse, partner of Washington, D.C.-based Luse Gorman Pomerenck and Schenk, some buyers wouldn’t mind losing certain types of customers.

Luse says that a bank may specifically want to target a new demographic in a different area, and wouldn’t mind trading one type of customer for another.

With regard to deposits, Luse said, some types of depositors just aren’t attractive to certain banks.

The acquired bank may have tried to increase its liquidity by beefing up the number of high-cost CDs or brokered deposits, but a larger bank acquiring them probably wouldn’t have liquidity issues, he said. They might decide, then, that the “expensive money” isn’t worth it, and price their rates encourage CD and brokered deposit holders to go elsewhere. Attrition, in that case, isn’t all bad.

“There are definitely cases where the buyer will say, ‘I don’t really care,’” Luse said.

As far as undesirable customer attrition is concerned, analysts believe a larger national bank from outside the region will lose more people than, say, a community bank from the same area, said Stephen J. Coukos, a partner with Chu, Ring & Hazel.

In fact, some community banks love it when a large institution buys up a nearby competitor, he said. When a Banknorth or Bank of America takes a competitor or even just a competitor’s branch, that still-standing community bank scoops up part of the exodus.