It's a curious choice but probably has some resonance with Republican voters where brand loyalty is a must, and the canonization of Saint Ronnie was declared years ago.
However, Ron Paul's own invocation of the Reagan legacy --whenever it suits him, it seems-- is the more curious aspect of the ad. From a factcheck by Newsweek in 2008:
From Ron Paul Web site: "Ron Paul is one of the outstanding leaders fighting for a stronger national defense. As a former Air Force officer, he knows well the needs of our armed forces, and he always puts them first." – Ronald Reagan
Paul's embrace of Reagan's legacy represents a significant change of heart. Actually, it's the second time that Paul has changed his mind about Reagan. After endorsing Reagan for president in 1976 and again in 1980, Paul became disenchanted, leaving the Republican party in 1987. The following year, he told the Los Angeles Times:
Paul (May 10, 1988): The American people have never reached this point of disgust with politicians before. I want to totally disassociate myself from the Reagan Administration.
Paul's disaffection started early in Reagan's presidency. "Ronald Reagan has given us a deficit 10 times greater than what we had with the Democrats," Paul told the Christian Science Monitor in 1987. "It didn't take more than a month after 1981, to realize there would be no changes."
Sometime between 1988 (during Paul's run for the presidency on the Libertarian Party ticket) and 1996 (when Paul, running as a Republican once more, successfully ousted an incumbent House member in a GOP primary), Paul once again embraced Reagan's legacy. The New York Times reported then that Paul had used the longer version of the Reagan quote in a videotape sent to 30,000 households. According to the Times, Reagan's former attorney general, Edwin Meese III, flew to Texas "to insist that Mr. Reagan had offered no recent endorsements."
We were unable to document Reagan's endorsement of Paul. When we asked the Paul campaign for documentation, a spokesperson told us that the campaign was "a little more focused on positive things." The Paul campaign did not provide the Times with a date for the quotation in 1996, either.
So Ron Paul continues to use Reagan as necessary, with a quote which may or may not have occurred, or simply been a rubber-stamp endorsement presidents often make on behalf of congressmen, to keep trying to get elected by republican voters.