Agreed. Most people don't use them because overall, leaving your black tank full instead of dumping at the end of your camping trip is described as a bad idea. I'd venture to say 95% of the people on this forum dump at the end of their trip or within a few days. They are afraid of buildup, smell, causing other problems since thats the advice of 95% of the people when this question is posted.
If your not going to dump the tank in after a few days of camping, it doesn't need the breakdown stuff. Expense flushed down the toilet basically. Plenty of water keeps the dreaded pyramid of poop from building up and also helps break down everything.
SOme that use the chemicals do so for smell control, some do it because the place they bought their camper told them to, some do it because someone on here advised them to.
It's a personal choice which advice we all take to heart and try. I have not seen anything scientific in any post concerning black tanks. Most of the posts are strickly opinion.
And you know the opinions we agree with are the truth, the ones we don't are just propaganda ;-)