I got a very nice lv 3 rune (pic below), is it really worth using if I have 0/3 in this talent?

I think it's worth it considering you have 0 talent in holy power. The first talent point will add 6% to your attack where-else the subsequent ones only increase it by 2% each. So it acutally works better with 0 talent.

When you stack holy power to 3 times, that's 18% increase in attack. Convert those attack into heal with sympathy and you'll generally get around 50-100 more heal effects. Not alot, but considering you get all these by 1 rune, that's decent.

Im quite sure I have read somewhere that without a single point in a talent, the talent that the runes were supposed to increase will not activate.

It will only increase base dmg of offensive skills, so divine strike/enigma/ice blade/judgement to 106/112/118% and frost to 156/162/168%. So it's nice if I'm not using sympathy (trash mobs, easy bosses, LCS, GL, GT), it could be even nice for my offensive playstyle

The descriptions are incorrect. Holy Power increases our attack attribute directly instead of adding base damage to offensive skills. You can actually see your attack attribute increase on the character stat screen when holy power is in effect.

Furthermore, Holy power's effect is lost when we successfully casted an offensive skill, not when we're being struck (OK, I noticed they corrected this part of the description. Nice )

You're right, Holy power can be useful for offensive playstyle too. Its strength is that it can be utilised both ways, offensively or defensively.

That is for active talents only, so you can't for example get divine shield, if you don't already have a point in it.

For passive talents (most of them) it works just fine.

I see, thanks for pointing that out.