The reasons I add a THF to a party are ... well, none. Everything a THF can do for an XP party, some other job can do better.
Call me stubborn, but that's exactly what I think should be resisted. Take a completely peaked out Axe/Axe or G-axe war, Mnk/Nin, or any other DD other than Sam, probably, and pit them against an equally peaked Thf, no relic considerations. In all honesty I think the Thf parses higher. In my experiences the only way a War or Mnk should outdamage a fully-peaked, skilled Thf is by going Mnk/War, or having Ridill. And even then, at camps like Birds and lolMire the Thf should still come out on top against even those DD's.
So damage-wise, I don't think there is much argument. Of course, if we're talking about poor Mnk, War, Sam, w/e vs poor Thf, the former classes will do the most since they're easier to play and easier to max. For skilled players and peaked out equips, I don't think this is the case. As far as utility goes, the only real utility I can see is an argument from vokes. C-stance Mnk/War and first-voke seigan Sam are kind of like...really rare exceptions to this, so the only classes that I see as having a definitive utility are Nin and War under most circumstances. And then, in conditional situations I think adding TA to the party is more
of a boost to the party than another voke. Ever pt'ed with a 4 DD party with an E-Body Ridill, thf, and 2 mediocre DD's? I have done a few. What's a second voker going to add to the party that wouldn't be better than trick attack? Edit: other than voke an add that the thf could have taken off of the puller with a single swing, to which A+ evasion will be quite nice while melee attention goes back to the mob that the others are fighting.
I mean, compare Mnk/Nin to Thf/Nin, for example:
Mnk/Nin - Chakra, conditional counterstance, hundred-fists, high hp.
Thf/Nin - Feint, aura steal, trick attack, high evasion.
This isn't a comprehensive list, but some of the more useful utility traits of the jobs that they can bring to the party. Make further comparisons with other jobs, and you can see that the real definitive utility-booster is having provoke.