^
You're right, Nemoy was pretty bad too.
But Nimoy didn't take himelf seriously as a recording artist. He knows he sucked, hell, he couldn't keep a straight face when it being recorded for some TV show (Smothers Brothers I think it was).
Shatner actually took himself seriously as a "singing" artist. Then again, he also took himself seriously as an actor for 45 years before he realized he sucked and had a wooden leg, but that's why we loved him.