I agree with you that creating the soul good/evil dichotomy was really limiting. Even if Joss had stuck to his guns and NOT introduced complex demons with shades of gray, he'd still have to face the fact that a soul does not always make a person good. Even if we'd never seen Faith, or Warren, or any other human who did evil things, we still know that in the real world, there are rapists and murderers and all kinds of evil people with souls. How do you justify that a soul automatically makes a person good in the face of all those examples to the contrary? Including demons who are not strictly evil introduces the same kind of moral gray areas that already exist among humans. If there can be evil humans, why can't there be good demons?

So, to answer the question, Should Joss adhere to black-and-white position he proclaimed in the first episode of the show? I would say no, because the position was untenable to begin with. Maybe, as others have mentioned, it makes a decent starting point for the characters, given that they are very young and have simplified world views. But it would seem completely false and the characters would appear terribly naive if they'd continued to uphold those simple beliefs. (And, in fact, they come off as rather ignorant when they DO express those beliefs on occasion later in the series [usually related to Spike].)