I think you have valid points, but are missing out on some aspects of an endorsement.
For example, Ted Kennedy's endorsement of Obama.
Besides handing "The Kennedy batan" to Obama which IS important to some people.
Kennedy is universally considered to be one of the main power brokers of The Senate, with a proven ability to reach across the aisle to work with Republicans and get things done.
He has a huge staff of talented and connected advisors who he has put at Obamas disposal.(strategizing,consulting etc.)
Kennedy has also actively campaigned in area around the country where The Kennedy name still has magic and influence.
Finally, with need to rack up super delegates being so important this year, I feel highly visible endorsements from power brokers like Kennedy and Richardson will help tilt other super delegates to the side of their candidate.