As I write this story, my wife and daughter are enjoying a long weekend at Miraval Berkshires Resort & Spa. It's my wife's third Miraval stay in the past two years, and we didn't pay out of pocket for any of them.Dropping them off at the airport got me thinking about one of the most overlooked perks of frequent travel: Many elite benefits can be shared.Road warriors and business owners who spend a lot on credit cards can garner the most elite lo…