Reading the wiki for Spiteful Talisman I noticed the note that says that it actually doesn't increase the range nor the cooldown of the skill Reaper's Touch. Since the Focus has only 2 skills that would mean that "Spiteful Talisman" works only on 1 skill? That would seem rather useless to me. Can a high level Necromancer confirm this please? I was looking into making a Necromancer using a Focus with this trait to increase the cooldown on "Reaper's Touch", but I'd like to know whether this is possible.
This was a bug, but it has been fixed towards the end of 2012.
I believe the wiki was updated erroneously later to add the bug comment back in, but from checking just now it's been removed again. For clarification, you may wish to look at the history comparison of the entries for Spiteful Talisman and Reaper's Touch.
You're absolutely correct, though, a trait that only affects one offhand skill is quite useless, unless we're talking about an extremely powerful skill. Even with it affecting two skills now, I'd question its usefulness, but then, I suppose not all traits are created equal. (And often you only realise the power of some traits when you figure out how to best combine them with others.)