On paper I like your idea. But the problem is that a prophet is easily 20% of your total points (average games are 2-2,5K a prophet usually costs in the 400pts)
20% is a lot. This is one of the down sides of the loa list. If you are taking a prophet you have to make the most of him or you are wasting to much points. This seems to be the main reason most prophet builds are roughly the same. There simply aren't any good other options.
Deamonsmiths on the other hand are excellent 'support' chars. They boost your war machines, are lvl 1/2 wizards and fight just hard enough to stop/slow down annoying (ethereal) skirmishers. If you want to have support troops take 1/2 of those and kick ass with the prophet (no pun intended)
Well I've got to disagree it's a waste of points using a Prophet in a different way, just because he's not near the front getting beaten up in combat! If anything I'm making more use as I'm using him for war machine reroll and skirmisher protection behind the lines!
In a unit of Infernals over say three turns of combat in a game he's going to kill about 10 rank and file if he's lucky. Maybe in a challenge he might come out okay and kill a hero or crap lord as well but it's risky for him.
Also I wouldn't say the Prophet kicks ass, he is better than most other lord mages but he's isn't fantastic.
I'm sure there's people out there that use Prophets in different ways to the usual?
What about a mounted prophet with that 100pt magic item that cause panic and things to blow up?! fly into the middle of the enemy and unleash the fury!