Finding the balance can be tough, though. If you completely throw out XP for gp's and just go with monster XP, the party has to kill something on the order of 500 orcs just for everyone to reach second level, which motivates players to hunt down and kill everything they come across (which can be just as problematic as having them look everything they come across).
Plus, greed is fun to punish! If there's XP in gold, that giant ruby eye in the statue becomes a lot more tempting, and the traps and curses that protect it become a lot more fun to set up.