^which brings up a good point to share about ammonia formation and people who are wondering if they have it.
Two conditions are required before this bacteria can be a factor...
1. The FC must be at zero, and typically for much longer than a few days.
2. Your CYA would read as zero, or at the very least, dramatically reduced.
If you open a pool with zero FC, it doesn't mean you have ammonia.
If you one a pool with zero FC AND zero cya...it still doesn't mean you have ammonia...it just means that the conditions were right for the conversion to occur if the bacteria is present. The majority of the time, the bacteria will not be present unless there is also a lot of debris. Technically speaking, the conversion is supposed to be anaerobic...but obviously, since it still happens in a pool, there's more going on than we, or at least i, currently understand
With that said, since the test is SO simple and you dose up on opening anyway, I feel its not a bad idea IF one opens at ZERO FC and CYA to do a quick check before adding cya.
Other things that cause chlorine to disappear quickly include ascorbic acid, pool antifreeze in sufficient quantities, etc. plus chlorine will naturally dissipate in the sun with little FC, so losing 50% on a swampy pool in a few hours also does not signify ammonia necessarily.
That's why the quick succession of application/test is important...it shows the rate/speed of loss, which is dramatic in the case of ammonia.