I give my quail birdseed as treats too, and have also wondered whether it could be a complete diet when supplemented with bugs, etc. This is an interesting thread.
The way I feed birdseed is by offering it as a treat only, generally tossed on the ground (or in their sandboxes if they're up on wire) by a handful or two a day. They have a great time picking the treats out of the dirt/sand/whatever, so they get a little exercise and mental stimulation in the bargain. It's not enough to fill them up, so then they still eat their crumble.
If your birds have lots of access to forage (i.e., plenty of space on the ground where they are digging for bugs all day, with access to green weeds, and only during temperate seasons when bugs and weeds are plentiful), you can be a little "looser" in your feeding guidelines. They will still thrive best when given access to a healthy packaged food (or nutritionally balanced home blended feed--most people don't have the resources to accomplish this) and only minimal access to "treats." Most people, however, can't afford to give quail adequate room to, essentially, "free range"--because their need for protection is so much greater than for ducks and chickens and the "free range" area needs to be completely enclosed... which is prohibitively expensive for most.
That having been said, the comment regarding birdseed being unnatural in large quantities makes a lot of sense. A ground bird like a quail would have access to seeds only when they fall to the ground, and only in season, and only when they could get to it first. Bugs, on the other hand, live in every square centimeter of the soil they walk on, and are a naturally huge part of their natural diet. And, just as humans crave fats and sugar because they are scarce and valuable in our "wild" (lol) state, it makes sense that quail would crave what is scarce and valuable in theirs. And that it would be likewise unhealthy for them to have unlimited access to it.
Thanks for the thread. I'm interested in what others have to add, too.