I haven't tried any lesser-used subs in party situations before. I probably would have given DRK/DRG or DRK/RNG a try back when my DRK was in the 20-30 range (long, long time ago), but neither of those subs were leveled for it anyway.

I recently leveled NIN to 30 to sub on my RDM, but that's purely for social purposes, although I did get an invite when I was running around as RDM/NIN yesterday.

Honestly, I'd rather get parties with subs that are optimal for my main than wait around trying to get a party with something like RDM/WAR or RDM/THF. The only sparsely used RDM sub I think I'd ever try in an exp. party would be RDM/BLU, but, as I understand it, that combination, like RDM/NIN, is mainly for solo purposes.