Yosharian
Arcane
OK so there are a couple of things at play here, first of all by making him an MT we sacrifice his domain power progression and some Cleric spellcasting ability. Harrim's two domains are Chaos and Destruction. Harrim has 14 Strength so Destructive Smite is of limited use, still not too bad. Destructive Aura is nice because it can power up our allies' attacks. Touch of Chaos is rather nice for boss fights, but perhaps you'd be too busy casting other spells (but in any case, Touch of Chaos doesn't scale at all, so it's equally good at Cleric 1 as it is at Cleric 20). Chaos Blade is basically pointless since most enemies won't be Lawful.Yosharian I saw that in your build guide you turn Harrim into a Cleric of Groetus 3/Empyreal Sorcerer 7/Mystic Theurge 10 thing to try to "salvage" him since he has bad stats. I like him as a character so I'm gonna use him. I saw that Nerd Commando just simply turns him into a Druid in his attempt to "salvage". Is the power difference between your choices that big or can I go with whatever?
So overall the Domain abilities are nothing to write home about. Furthermore, I tend to build with an emphasis on limited resting, so I dislike abilities like Destructive Smite that have limited uses per day. All of Harrim's domain abilities run contrary to this philosophy.
So with the Domain abilities being limited, all we're left with is the Cleric spellcasting, since Harrim's ability to Channel Energy is pretty wack. Well, I'd much rather have an MT with slightly weaker Cleric spell progression and pretty good Sorcerer progression, with spell slots lasting for days.
Finally, I wanted something a little different to just standard Cleric, to make Harrim stand out from my existing Cleric of Erastil build. It was more this desire for a unique build, than a need to have something which copes well with Unfair difficulty.
I hope this explains why I went for MT, but honestly if you want to stick with standard Cleric, it's all good.