If a Persistent Divine Power gave a cleric all the feats and equipment of a 15th level fighter, then I would worry about it. As it is, if a 15th level fighter goes into battle with only an 18 (modified) Strength, and if that fighter has used all of his feats on things like Extend Spell and Persistent Spell, then that fighter deserves to feel weak, because he is an idiot.
By 15th level, most fighters will have a buffed strength over 18. They will have at least 9 more feats than the cleric and they will not have had to use any on caster abilities. They will have combat-oriented equipment and they will not be expected to be healing the rest of the party during a fight. If, given all of those advantages, you cannot outfight a cleric with this spell going, then you really aren't pulling your weight to begin with. Don't weaken the cleric, tell your fighter to get his act together!
By 15th level, most fighters will have a buffed strength over 18. They will have at least 9 more feats than the cleric and they will not have had to use any on caster abilities. They will have combat-oriented equipment and they will not be expected to be healing the rest of the party during a fight. If, given all of those advantages, you cannot outfight a cleric with this spell going, then you really aren't pulling your weight to begin with. Don't weaken the cleric, tell your fighter to get his act together!