I beg to differ. MNK defenately have a very good use for NIN as a subjob towards the endgame section, so does SAM and BLM. But I do agree that sometimes /NIN is way over-rated.
Don't go RDM/NIN in party situations. I almost laughed when I tried to invite a RDM/NIN, telling her to sub WHM (because she had it) -- but no, they only wanted to melee, which was absurd.
For RDM, overall, you'd want BLM and WHM for party situations, DRK and NIN for certain other situations, as subjobs.