There’s a specific reason why 18 owners don’t like to use the stock water pan and go with an alternate diffuser.
The stock 18 pan is way deeper than it needs to be and overly compresses the fire compartment.
But most of us still use something most of the time — flower pot saucer, pizza pan, firedial etc. And there is a lot of variation among 18ers on whether and when they cook with or without water.
I think no diffuser is mostly done for high heat cooks — like Chris’ hot n fast chicken. Or uds pbc style hanging ribs.
I think the pans on the 14 and 22 are relatively flatter and so don’t not get in the way so much. But I think full pan, dry pan or no pan are all good options for different kinds of cooks in any size of wsm.