I fully agree with what Bad King says. Regarding oral traditions, I would point out that it can be a very reliable and robust way of preserving the history and traditions of a civilisation. It all depends on how seriously the community regard this method of transmission. Many cultural groups in our own world had an oral tradition and specific individuals in the community were charged with maintaining it; just as the Keepers and story tellers do in the Dalish. It was only when the cultural community died out that the lore was lost because there was no one left to care about preserving it. So long as the Dalish exist, so will their oral traditions be passed on. One of the reasons for the ten year gathering of the clans is to exchange lore they have gathered in the intervening period and catch up on developments with the other clans. So in a way this does allow their culture to evolve and grow whilst retaining their cultural identity.
Based on what they remembered down the years and what they discovered in their travels, the Dalish got an awful lot of the history right, not wrong. The only bit that was incorrect was that humans were responsible for their downfall but then the humans themselves (Tevinter) had encouraged that view and Abelas did say that the humans were effectively carrion creatures feeding on a corpse; well carrion creatures can be very aggressive and since the surviving elves were part of the corpse, even that memory isn't totally wrong.
Why is a culture only valid if it builds things? There have been numerous nomadic cultures in our world that didn't build things. The ancient elves built things and the largest monuments were built on the backs of slaves? That is also true of Tevinter. Same could be said to be true of the "great" civilisations and empires in our world. So may be people are better off generally when a civilisation isn't dedicated to building things just for the sake of it and to reflect their sense of self importance.
Morrigan and Averline are the exceptions that prove the rule about atheism and they are more truly agnostics. The whole argument that was being made to us throughout DAI (being shoved down the throat of the Inquisitor whether they liked it or not) was that the Chantry was a unifying force throughout Thedas. Even Dorian, who dislikes the Chantry, says he finds it comforting to think that the Maker is watching over us (even though the Maker seems to do very little other than just watch). To my mind, if the Dalish beliefs are invalidated by what we learn of their gods, what of the Chantry? If you haven't read WoT2 then go to the Wiki and see just how different the Chantry version of history is from what really happened with Andraste. How few bits of the Chant of Light are even believed to have genuinely come from Andraste. How many were written down years after the event. Over the years bits of the Chant have been put in and then taken out again, mostly on the whim of one Divine or another.
The Avaar worship "gods" who are really spirits. They acknowledge them as such and are surprised when questioned about it. The dwarves revere the Ancestors and the Stone. The Dalish gods really did exist; they had particular areas of responsibility and it is these aspects that the Dalish follow, knowing from their own lore that the gods can't help them because they are shut away from them by the Dread Wolf. (All bar Mythal who seems to have no problem with the Dalish and frequently helps them - albeit for a price). This has been the case since the time of the Dales (and probably long before then), so why is it suddenly so idiotic of them to continue to follow their faith just because the same Dread Wolf claims their gods were power hungry and corrupt and deserved to be banished? You could just as easily show them the message from their gods that warns them against listening to the words of the Dread Wolf who will "offer advice that seems fair, but turns slowly to poison". Even if their gods ultimately became corrupt, even Solas seems to suggest they didn't start off that way and it would seem it is this aspect of them that the Dalish remember and reproduce with their Keeper mages as guides and leaders of the community.
On the whole it would seem that the Dalish, along with the Avaar, have the greatest degree of equality in their community, compared with the cultures around them. The Keepers do not lord it over the others; there seems no particular class structure; no nobles and peasants; their laws apply equally to all. There are no castless among the Dalish and so far as I am aware they marry where they choose; not arranged marriages for titles, power, prestige or material gain. Respected elders within the community have responsibility for different areas of it. I'm pretty sure no Dalish is ever forced to remain in the community if they do not wish to. They may be banished if they do not abide by the rules that govern the community but that is true of every culture in Thedas; in some you aren't even given the option of leaving and are hunted down if you do. That quote from DAO by Bad King just about sums up the reason why the Dalish live as they do and why this choice should be respected, not despised because it doesn't fit with modern notions of what a valid culture should be. Just because some clans have lost their way and fallen from the ideal, doesn't invalidate all the rest.
Saying that all the races would be better off not worshiping anyone at all just doesn't fit with the world they inhabit, since the writers have seen fit to make it a world where faith is an important facet of life. However, their faith is not the only thing that defines the Dalish.