"In theory--- assuming you or a guest doesn't pee in it--- the only thing going down a shower drain is water, dirt, and soap/shampoo. Perhaps the theory is that this stuff doesn't have the potential to turn nasty down in the bilge."
If so, it's theory based on a false premise..'cuz shower water is loaded with bacteria from body and hair oils and perspiration, soap scum and other things that help to turn bilge water into a primordial soup that makes a whole boat smell like a swamp or even a sewer. So by all means, install a sump!
And to keep that sump--and your head and sink drains too--sweet smelling and clean and the pumps free of hair and other clogs, you need to maintain it. The best sump and drain cleaner on the planet IMO is Raritan "C.P." It's a bio-enzymatic cleaning product that Raritan only markets as a toilet bowl cleaner. C.P not only destroys odor on contact, but the enzymes in it "eat" hair, soap scum, body oils, toothpaste, cooking grease etc...making it ideal for maintaining sumps and sink drains. A couple of ounces down the shower drain into a sump and/or a sink drain once a week when it can stand at least overnight (it can remain for a month or longer...there's nothing in it that can harm the system) is all that's needed to keep drains and sumps clean odor free.