You can upgrade community lots with all-in-one shower units and help avoid random stinky Sims wandering around.
The game doesn't recognize the all-in-ones as a replacement for toilets and sinks.
So if you want to get the benefit of upgrading your community lot (investment wise) without cluttering up you bathrooms ...
You have to build a basement with no access and put toilets and sinks in it.
Edit:
It should be noted that it takes Sims less time to use the all-in-one shower units than the toilets and sinks anyway.
A tip would be to upgrade the units to self-cleaning.
And ... If they ever break ... Have a Sim with the plumbing achievement fix them.
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