Good point, of course the "moral" reward (either for the good or evil guy) is always there and is the main reason such situations should exist throughout (eg. The Witcher).
The problem stems from the fact that there is no clear evil, so the Context is important, eg.
Imagine you (the robber) would be starving because of bad-food management or you are simply broke, your only choice is to rob someone or ask someone for help, you asked plenty of people and nobody helps you, you come across that rich wealthy merchant and he asks twice the price for food he normally would, would it be morally wrong to rob that merchant so that you could live? Is not a live more important than money? ;p
Aside that, such moral ambivalence is applicable to many quests, freeing the slaves of a slave driver might seem like a good deed, but in the constraints of a fantasy world (slavery might very well be legal, it sure as hell was quite a while perfectly legal in our reality as well) this might actually be a crime (And hence move your alignment away from lawful) but still a good deed (move your alignment torwards good)
Anyhow, DnD is the only game system that has a really well thought out alignment system, one that i think is applicable to the real world as well. One can argue that doing evil deeds though is punished too much in BG2 (prices !) Though i certainly never had a money problem, and i can not play an evil guy either, i always tend to fall torwads good chaotic, truly evil deeds are most of the time things i would not do even in a RPG.
That said, a game that does not supply me with the ABILITY to do such things (evil and good, lawful and chaotic) and True Neutral things (god i hate tree huggers) is not a good RPG .. ok enough brabbling from me ^^