To elucidate: DMM is one of the very few ways in which the Persist Spell feat, which all of us would like to use on an awesome self buff like Divine Power, can be made to work. At all. 6 spell levels increase? What were they smoking? Who's going to use a 7th level slot to persist a 1st level spell? Nobody, that's who. But with a little bit of investment - a feat, which a Cleric has to spare usually, and Turn Undead uses, which are best invested into something other than turning actual undead, anyway -, persisting becomes viable. Great.
I don't have to tell you how fantastic a buff Divine Power is. I hope. I mean, it makes you a full front-line fighter for all intents and purposes, and you remain a full caster, as well! Dual threat, thy name be Divine Power. Not to mention that DP combined with your other buffs will let you be a better fighter than the Fighter much of the time...
Now what's great about persisting DP instead of casting it regularly is the following: a) you don't lose a round when battle is joined, or can use that round for another buff, or battlefield control; b) you're already buffed up when surprised, which can be invaluable; c) no matter how many fights you're going to have in a day, or how long they take to resolve, or how cleverly your opponents use hit-and-run to let your buffs run out... your most important buff is up and stays up.
I don't see how you invest a spell slot. I'd say that in a regular adventuring day, persisting DP saves you three 4th level slots instead of costing one. Coz you use only one 4th level slot for your regular four encounters.
Look at it this way: Imagine you're able to cast 9th level spells. Wouldn't you like to cast one of them out of a 3rd level slot instead? Miracle, as a 3rd level spell? Sound good? What if I were to tell you that all you need to make that happen is spending a feat and 6 of your daily Turn Undead uses? I bet you'd do it.
Turn uses, btw, are cheap. Cha boosters or the Extra Turning feat (or classes that grant it) sound like too much of an investment to you? OK, just use a bunch of Nightsticks instead, or take a domain that offers a different kind of turning (the elemental domains, Scalykind, etc.), or both. I recently had the PCs encounter an evil Cleric with DMM: Quicken (not Persist, but still costly). He could easily quicken 4 spells for free each day. Nice trick, and it had my players gaping. That Cleric was only level 8!