
Drakli |

Okay... stick with me on this.
I'm not talking about a fiend who's half fiend, half another fiend.
I'm talking about applying the Half-Fiend template to an existing fiend to represent that it is more densely connected to abyssal, hellish, abbadonic(?) energies than its compatriots.
Think about using it instead of the Advanced Creature Template to create a demon who's a leader of men(demons.) You'll gain ability score bonuses (not as many as Advanced Creature, but enough to enhance the creature the way you need it) but also enhanced mobility through flight, and additionial spell-like abilities with which to play. There's bound to be some overlap, particularly with defenses, so don't be afraid to adjust the plus to CR. As an added bonus, increasing the creature's racial HD or giving it class levels may net additional spell-like abilities!
What do you think, sirs?

Fergie |

I have been known to add the half-fiend templates to just about every type of creature from ooze to unicorns. I find that adding templates to creatures they weren't intended for can create some great creatures. You just have to tweak the power level here and there.
I think you just created the fiend-and-a-half!

Jandrem |

Just a point, but are you talking about adding the 'Half-fiend' template to the Teifling?
Now, ass in the new 'variant' Teifling things from the latest adventure path and you will truly create some unique characters/adversaries.
So, who's up for the Asimar/Half fiend? ;)
Cheers!
I rolled something like this once, in a Greyhawk campaign where we wound up in Acheron. I had a character who had Fiendish and Celestial parents, and came out as a weird hybrid. The DM loved my idea, so we worked together to make character. We basically had to think of races and templates as building blocks, and just build up the character as I saw it in my head, so we used the Aasimar race, added the fiendish template, and added Demon Wings(a feat from Forgotten Realms).
Basically, her parents were outcasts because they chose a life of neutrality and fell in love. She became a Paladin of Freedom (CN- Unearthed Arcana variant) who fought for people's freedom of ideals. She had bronze skin, red eyes, gold horns, and red feathered wings. Statistically, she was gimped as hell. But, this was one of those characters that was fun just to play, with little regard to actual mechanics.

Drakli |

Just a point, but are you talking about adding the 'Half-fiend' template to the Teifling?
Nope, I'm talking about adding the half-fiend template to, say, a babau for example. It gains a fly speed, desecrate and unholy blight once per day and poison three times a day, can smite good once a day, and its ability scores bulk the heck up! It even gains DR 5/magic, in addition to its existing DR 10/Cold Iron or Good, which is just icing. Truly, is this not a fiend and a half, as Fergie so well put it?