Look, I know how it's important to US, as fans of the Star Trek franchise, for him to attend Nimoy's funeral... but when you get down to it, this is a REALLY personal thing. Nobody has a right to tell another person how or when to pay their respects.
Myself, I have never attended a funeral, and I don't expect to do so...ever. Not because they're meaningless to me, or because I'm insensitive. I just choose not to stand on ceremony and grieve in public. I absolutely MUST deal with these things in private. If you ever gave ME shit about it at a very vulnerable time, like some of Shatner's fans are on Twitter, they'd probably be putting TWO people in the ground. :P
I'm not suggesting that's what Shatner is dealing with. We don't actually KNOW what his true feelings and motivations are. I'm saying don't put so much weight on being physically present somewhere in order to pay your respects.
That said, there's another angle to this that I don't think many people really think about.
Personally, I never got the impression that Nimoy and Shatner were quite as like-family close as we'd all like to imagine. Undeniably, they have a famous, shared history, and they seemed to get along well.
But, behind the scenes, who's to say Leonard wasn't just this guy he worked a lot with over the years and occasionally meets up with to promote stuff together?
Being at his funeral might not be as important for him, personally, as it is for US. If that's the case, that disappointment is ours to bear, not his. That's not to say it won't be a terrible PR move. He'll definitely catch shit from his audience over it, if that's how things really are for Bill.
Anyway, the guy tried to get head of the wave by admitting his inability to attend via Twitter, and tried his best to improvise, staging a virtual memorial through his account.
There's not really much more you can ask, except for us to just move on.
UPDATE: Turns out he DID go. Hopefully on his own terms, and not because the disgusting media and incredibly rude fans were pressuring him to do it.