IMO, while a WAR post 30 makes a good, all-around melee, a PLD makes a better tank while a DRK or SAM makes a better damage dealer. It's similar to the reason groups look for a WHM to heal and a BLM to nuke, and wouldn't pick an RDM as first choice for either slot. Then again, RDM gets some nice abilities that *they* specialize in (debuffs and, later, Refresh), whereas WARs don't, really, so RDM's are sought after at later levels, as well.
The PLD can generate more than enough Hate to keep the mob centered on him, without needing a DS Curaga. They, too, get healing spells to help out in a tight spot. While they don't get group-effect spells, they do get VIT+ and DEF+ passive abilities, which means they take less damage - hence, specializing.
I hate agreeing with the Brady Games guide, but they say it best: the problem with WAR as a main job is that it makes an even better subjob.