This answer lists several snippets from articles theorizing why Shatner did not make an appearance during the unveiling of the NASA shuttle Enterprise. Half of them are less than positive and sound like Bill was a known jerk back then. I've only ever heard good things about him and am not sure why all that hate was thrown around.
Here are the excerpts in question (emphasis mine):
But where's is Shatner? He probably didn't attend, because he's too cool for school. Or maybe he was banging a NASA girl in the shuttle bay, who knows. But all is forgiven: At least he went back to NASA to record the last wake up call for Discovery.
If you don’t recognize them, that’s the cast of the original Star Trek television series, minus William Shatner who was apparently too important to show up. (Actually, I don’t know why Shatner wasn’t there, but it’s no secret that he frequently behaved like a colossal jerk during the ’70s and ’80s.)
Therin of Andor: Shatner made a habit of being absent from such high profile events, probably his agent holding out for an appearance fee. He was also absent from all the other actors' Hollywood Star Ceremonies (except Nimoy's and Roddenberry's), and more recently George Takei's wedding.
WHF: He was probably the only one with a job at the time and had to work.
So was William Shatner as bad as these people say he was?