I completely understand what you are saying.
The fact is, however, that under normal circumstances one can't go by the symptoms because they can mean anything. All these problems have the same exact symptoms, whether it's bacterial infection, stones or crystals, or allergy. Also under normal circumstances few cats really have a bacterial infection, their symptoms are usually the results of other underlying causes. So as a general rule, for people with healthy younger cats, starting with a complete urynalisis is always the best, safest way to go.