You know, this always makes me curious, because I've had nothing but trouble with this concept in the past. A lot of published adventures, modules, and even the bulk of a lot of custom made material often features humans as being the most prolific race. Usually, other races have certain stigmas, prejudices, stereotypes, or other shennanigans attributed to them, either true or untrue.
How often do DM's actually have their populace react logically to a group of demi-humans walking into a town that is nearly 90% human? It's easiest when the demi-humans are the old staples, like dwarves, elves, halflings, etc. However, I often see people list their party, and it's full of tieflings, gnolls, half-minotaurs, pixies, drow (that are totally good), and other more bestial races. Do DM's ever have people run in fear? Do mothers shield their children and usher them inside? Do groups of guards suddenly appear, shadowing the party as it moves through town? Do innkeepers frown and secretly up their rates as the gnoll barbarian slobbers on his countertop?
Sometimes I think a lot of parties fall in love with the idea of playing half-bastard spawn of dragons, but DMs basically just treat them as humans for all the world responds to their presence. I feel like that's really cheating groups out of potentially awesome RP, and also somewhat breaking an internal consistency in campaign logic. It also makes me wonder when people talk about how they can't understand why people would play a human, if that poster plays in a game with bestial races that are treated as human by the campaign world.
In such a case, I totally agree, why play a human if you can play any race you want and be treated as human?