He is more asking because certain grains cannot be steeped in conventional brewing, grains like Rye. The flavor will not come out as expected.
I wish I could answer your question fully asidrane, but we dont really know what malted and mashed quinoa, millet, etc taste like, so we have no way to compare.
That being said, I know that steeped grains will add some sort of flavor, so I think you should try it and see if it is something you like the flavor of. Remember that you can do so outside a brew (just put the grains in 160F water) and get the separate taste of just that grain. Like it? Add it!