I think it is a symptom of us humans feeling the need to neatly categorize the world around us, whether it be tents, mountains (Munros, Wainwrights, Marilyns etc), or even the heat of chilis!
I certainly fell into the trap with tents. I take a 4 season rating to mean that the tent will take pretty much anything that the British weather can throw at it, especially wind resistance. 5 season I would regard as being able to withstand arctic/expeditions conditions - a blizzard on the Cairngorm plateau, for instance. But I'll not be going up there in those conditions.
But that is not the whole story. I also want the tent to be easy to erect, relatively light, and to have good ventilation control so that it can also be used in the summer. Many 4 season tents either have too much mesh on the inner for winter use, in my opinion, or have too little for summer use. I'm a big fan of double layer doors (mesh + solid) - make the tent feel lest claustrophobic too, which is important if you are holed up in a small tent on a midgy September evening, but I like to be able to seal the inner up tight in winter.