MLK was good on speaking with inspiring delivery, and on the Vietnam war (antiwar). The early boycotts he helped organized were against state-imposed segregation, and as such were originally opposed to the use state force to perpetuate racism. Because of his socialism he and the civil rights leaders of the time did not maintain this anti-state stance once the federal laws began to roll in the other direction, and imposed integration and quotas on the nation.
In all, he is an historic figure in moving the country past the absurd era of seperated water fountians, and as a representative of that era merits the holiday (admittedly it would be better to call it something other than his name). His biographical flaws (womanizing, plagarism, communist buddies) put him in the same boat as your average flawed Founding Father (who were often slaveowners, violators of Indian rights, or inconsistent in following the Constitution). If we balance their merits/demerits yet honor the founders, we can do the same for the King.
Thanks for distracting us from the silly religious stuff. 
~hug~
