My thoughts about a lot of stuff in the soap has nothing at all to do with stuff going down the drain. Good grief, with all the stuff we wash off of our dishes that goes down the drain, and emptying our mop water that goes down the drain and a million other things that go down the drain (not to mention what gets flushed (yuk), a tiny bit of lavender buds or corn meal isn't going to grind the sewer system to a halt. (You would probably have to use a whole lot of bars of soap with corn meal in them just to equal what goes down the drain when you wash one pan that you baked corn bread in.)
But, my thinking is, when I see that people put sugar, or honey or many other types of organic stuff in soap....... when I get something sugary on my hands, I go to the sink to wash it off, not smear it on. This can go for dozens of other things I have seen listed as ingredients in soap. (I spill coffee on myself, I wash it off.....) I mean, really, we use soap to clean stuff off of us and with the exception of some emollience to keep our skin nice, and some foo-foo to smell nice, do we really want to be smearing more stuff on than what we're washing off? And in addition, organic material decomposes, fact of life, plant stuffs rot given time. So I would think that organic materials in their natural unchanged state in soap would contribute to spoilage.
Maybe something that I don't understand yet happens to the chemical composition of all this stuff in soap so that what ends up on your skin isn't the same as what came out of the original bottle or bag or whatever, but I guess I have a hard time getting my mind around all this stuff being rubbed on me that I would normally be washing off of me, unless that stuff has been proven to be specifically beneficial to the skin. But if it's just for aesthetics.... maybe it doesn't belong in something that we're going to be smearing all over our bodies.
I started out soapmaking trying all kinds of additives just because it was all so interesting and exciting, but the more I think about it, the more convinced I am that I should just skip all that stuff and make a nice basic bar of soap. (Giving up the ground lavender buds might be hard though!! And the skin benefits of collodial oatmeal and goatsmilk are supposedly well-proven, so I can't say there aren't exceptions.)
But I can't really market my "soap" based on the benefits of any of these additives anyway, as I am determined to stick strictly to making only soap, without alluding to any properties that might have it classified as a cosmetic, so I'm probably wasting my time with it all.