Basing it on the research I've read during my undergrad.
Candidates like endorsements because its free publicity, the same way getting on a media television show is free publicity.
The value of endorsements are in the way that the candidate uses them. For example, in a Texas commercial, the Obama camp claimed that he had more credibility as a leader, then used all of the major Texas newspapers that endorsed him as proof.
Richardson's endorsement might help during the general election in that state, if he uses specific advertising for it.
Most endorsements, however, do not have much effect for a couple of reasons:
1.) They are forgotten quickly.
2.) They are said once, then not repeated (for the most part).
3.) Other sources of information are generally more important to voters: commercials, news broadcasts, etc.
Of course, there are exceptions, and the big exceptions are extremely large companies with lots and lots of employees, national news media, and unions/interest groups.