Perhaps I have my lore wrong, but I was under the impression that 'demons' were merely spirits whose single focus was something negative rather than something 'positive' like Justice (and that, as DA2 demonstrates, any spirit is essentially the same thing as their one-track mindedness is very destructive).
They both are overly negative to the living world, its just someone are outright malevolent and others are too caught within their bailiwick to understand mortality.
There is nothing in Dragon as a "good spirit" its just that you have some that are more malevolent then others outright. Some try to foist the opinion that time within the living world or mortals themselves transfer negativity and change these beings from positive to negative, but in truth? Demons are Demons, and Spirits are just as bad. They are singular focus constructs after all, Just because a spirit is one of Wrath or Faith doesn't mean it won't go on killing spree's or possess anything in its sight.
Then you have this conception that spirits do not seek to posses mortals and do not try to devour their essence like demons do, That is more or less true but it still occurs, just for different reasoning. After all it appears most "spirits" Unlike demons do not possess the knowledge of how to possess, it seems once they gain that they actually go out and do it when the time to do presents it self, This was evident in Wynne.
Overall, It doesn't matter if you buy into them being of the immaterial or the Maker's first children, they are all bad news.
Its in essence similar to the Daedra and Aedra of TES.
You have "Benevolent" and "Malevolent" spirits but both view mortal as little more then either things of pity, amusement or what have you.
No real benefit to have from trucking with either.