Of course, Gerald Ford served only very briefly with President Nixon as well. President Bush (41) was selected as a running mate by President Reagan, served eight years with President Reagan, campaigned with him, etc. If someone can't distinguish the difference in the situations, that's pretty obtuse.... The reason Clinton spoke at Nixon's funeral was out of the Nixon family's respect for the office of the Presidency, which he then held. Bill Clinton, I guess, cannot possibly imagine that it was his office, rather than his magnetic personality, hypnotic speaking style, rakish personal charm, etc., that led to his role. Of course, respect for the office of the Presidency is such a foreign concept to the Clintons that it is no surprise he can't grasp this issue now.
The point is that Nixon and Clinton has more than just a casual ex/current Presidential relationship. Clinton has no connection to President Reagan.
If he had respect for the Reagans, much less for the office of the President of the United States, he would remain in reverent silence, even if his feelings were hurt.
What kind of person would want to speak at someone's funeral when they're not a friend or even a close associate? What possible motivation could someone have, other than being seen?