No Judgment Here, But Why Mitt?

Mitt Romney recently endorsed John McCain in the Arizona Republican Senate primary against former Congressman J.D. Hayworth. All opinions of McCain, Hayworth and Romney put aside, why does such an endorsement ever need to happen in the first place?

Do Arizona Republicans really need the opinion of a former Massachusetts Governor? Even if Romney sees himself as a national party leader, why get involved in primaries? Is Romney doing anything but just further going out of his way to link his name to the failures of 2008?