The main use of the Half races becomes apparent when your game includes racial bigotry. Frequently this can allow for some interesting roleplaying sessions. In a campaign I am playing in, 100 years ago a massive orcish invasion was barely pushed back. As such, many people don't trust half-orcs. In the game, half-orcs are charged more for stuff, room, food, and generally distrusted. In the same game though, Half-elves have absolutely no bias. This is because the Elves are a declining race and consider any child of elven descent a blessing. Plus.. they are pretty.
*note, I am not condoning racial bigotry.