Well, in 2nd Edition, demihumans were at least as superior to humans as humans tend to be superior to other races in 3.5. If you're going for optimization, it's often difficult to justify not being human in 3.5, and similarly in 2nd Edition, without level limits, it can be difficult to justify not being an elf, or a dwarf, or even a gnome or halfling. So there is some reason for that statement, it's not entirely baseless.
However, humans can dual-class. If you've got the stats for it, it's flat out superior to multiclass, because it allows you to stop dumping xp into a class once you have everything you want out of it. A Fighter/Mage dual-class can level fighter up to as high as they want to gain benefits from fighter from, then dual-class out into mage, and never have to spend xp on Fighter again. 7, 9, and 13 are great drop-points. An elven Fighter/Mage requires 7,500,000 xp to get to 20th level as a wizard, but a human Fighter 13/Mage 20 only requires 4,750,000 xp. The latter will have a THAC0 of 8 instead of 1, but otherwise won't have missed out on much that the elven Fighter/Mage has. When the human has earned a total of 7,500,000 xp like the elf, the human will be a Fighter 13/Mage 27.
Ironically, this seems like an ability far more suited to the long-lived races. An elf is likely to spend a human lifetime perfecting her skill at thievery, then set that aside once she's rich, practicing swordplay instead and becoming a famous warrior...then set that aside to focus the remainder of her life on mastering the secrets of the arcane. While a human seems much more likely to want to do all of those at the same time. It's one of the funny things I always thought to be backwards about 2nd Edition. Love a lot about it, but the racial class and level limits always left me scratching my head. Things like elves (renowned for their music and poetry) not being able to be bards.
In any case, if the character has the stats for it, being human for dual classing is usually worth it. The stats, of course, are the big limiter: you need 17 in the prime requisite of what you're dual-classing into, and 15 in your previous class's prime requisite. My own stat rolls won't let me dual-class unless I somehow increase Int by at least one point. Still, I would suspect that there's enough reason to play humans purely for the advantages of dual-classing, although the full strength of it only comes into play in a long-running campaign. Of course, racial level limits will also only become noticeable in a long-running campaign.