The normal method for preparing quinoa is to boil water, drop the quinoa, wait until it "sprouts", then strain.
My question is about the last part: the straining.
Suppose you are making a soup (lentils, bean soup, ...) and it turns out there is too much water. You do not want to discard liquid, and you do not have patience for reducing by simmering. Is dropping quinoa in, partly as a method for thickening, reasonable?
In other words, what is undesirable about the water one normally boils quinoa in, for one to discard that water? Is there anything undesirable?
Update (Long comment to GdD's nice answer and pointer to saponin)
I didn't wait for an answer. I ran two experiments (on only myself). The result on both occasions is that quinoa is indeed an excellent soup thickener, on account of the amount of water it absorbs. But on both occasions the meal required a few (5 the first time, 2 the second) tablets of Tums to neutralize the stomach acidity (a rare occurence for the foolish volunteer in question). This doesn't establish that saponin, or even the quinoa, is the cause, but that's perfectly plausible. Incidentally, on both occasions the quinoa was carefully rinsed in a fine mesh strainer. That didn't help.