This ability is pathetic. Let me list the problems.
1) Despite being a 1st level domain power, it is useless at 1st level since it grants a bonus of 1/2 caster level.
2) It is a standard action. For such a paltry benefit, a cleric will almost always have something better to do in combat.
3) It is an enhancement bonus, so it doesn't stack with magic weapons. A fighter with a +3 sword would only get +2 to damage if a 10th level cleric granted him this "boon."
4) It only applies to one attack. What this basically means is that at low levels it is not worth the time because a standard action is equal to an attack which is a valuable commodity at low levels. At high levels it is still not worth the time because it precludes iterative attacks which probably have a better chance of increasing damage potential.
My suspicion is that this ability was gimped with the desire to err on the side of caution. Understandable. However, I can't see this ability ever getting substantial use. Meanwhile a cleric with the Travel domain gets to telehop around the battlefield. Even the healing domain power will at least get used.
How to Fix
Make the damage bonus equal to the cleric level, make it an untyped bonus, and let it apply to any weapon. No character can benefit more than once per day anyway. It's better than making everyone carry a dagger just because that is your deity's favored weapon.