Because it makes quality constantly drop without a chance to raise it. When you have 50 units of 10/10/10 fodder and then attempt to drop 50 carrots of quality 20/20/20 you will still end up with 10/10/10 fodder, because one carrot is not enough to raise the quality and it's rounded down to 10/10/10. But put a single carrot of quality 9/9/9 and you end up with 51 units of fodder 9/9/9.
We've been throwing some leftovers from farming into food troughs. Most of it is around average quality of 10. But there were pieces like 1/7/13 etc.
The result is we have 4 troughs full of ql 1/2/1 fodder ^^
I think containers with volume should keep the decimal part of quality for the purpose of calculations when mixing.