For every Tiger Woods, there seems to be more than one Michael Phelps.

Why do brands keep trusting celebrities to represent them?

Is it time to retire the strategy of “borrowing equity” through endorsement deals? Historically, some celebs (like Woods, Bill Cosby and Michael Jordan) can be counted on to manage their image well, but obviously some are time bombs. Who knows when they will go off?

The dumping of Michael Phelps for “behavior … not consistent with the image of Kellogg” causes me to wonder again why any brand is willing to take the risk. Why place hard-earned brand equity in the hands of someone who doesn’t value it?

Celebrities are, after all, human beings, and therefore likely to err.

They may go ballistic like Christian Bale.

They might launch into a politically incorrect tirade, like Mel Gibson, Michael Richards and Don Imus.

They may be caught up in substance abuse scandals, like Barry Bonds and Floyd Landis.

Or convicted of animal cruelty like Michael Vick. Sorry, Nike.

Remember when OJ Simpson represented Hertz Car Rental? And Robert Blake represented STP Oil Treatment.

Some celebs rebound, some don’t. Martha Stewart is back. Kobe Bryant is mostly back. Alex Baldwin seems to have survived public outrage over the verbal abuse of his daughter. He recently starred in a Super Bowl spot for Hulu.

But with paparazzi and citizen journalists everywhere, celebrities are going to continue to be captured being human, whether cheating on their partners or their taxes. Why risk it?