The 'downsides', if we can really call them that, simply won't be THAT extreme....
If the downsides aren't that extreme, why not play evil?
I would suggest that the answer, boiled down, will amount to: the downsides *are* that extreme. Because it feels crappy making evil choices sometimes. It's painful to hurt innocents and not help when helping is within your power. Not to mention the in-game sacrifices.
