If all various odor blocks noted above work properly all the time, and nothing ever goes wrong when dumping tanks, and you never, ever make mistakes when dumping tanks and never spill even one drop of waster water on the ground ever....Then, yes, in that 'perfect' world/utopia, black tank chemicals are not needed.
However, I don't live in that 'utopia' where nothing ever goes wrong, haha. The world I live in, things occasionally do go wrong. Vents get clogged, water in P traps evaporates enough to allow some odor into the RV, hoses or hose fittings wear out and may leak a drop or two before I can correct the issue. THAT, is why I use black tank chemicals (Odorlos). They minimize the smell, when (not if) something goes wrong or doesn't go exactly as planned.
Your neighbors will appreciate you taking such precaution, too, when something goes less than perfect and allows some of your holding tank odors to escape (like maybe when a sewer hose has a slight leak, isn't attached securely to a fitting, etc.)
I don't think its as much about assisting with break-down of solids, cause as already said, that happens at the same rate regardless of chemicals. Its more about minimizing the odors for the times when something doesn't go exactly as planned.
Will and Cheryl
2021 Newmar Baystar 3014 on F53 (7.3 V8) Chassis ("Brook")
2018 Jeep Wrangler JK ("Wilbur")