It is a personal thing I feel, as there are many angles which this argument may take. It's better for a beginner or less confidant paddler to have it but not need it, and use it only when necessary.
I however prefer to paddle a narrow, hard chine, deep V, fish form greenland style kayak. I prefer they way the boat handles in rough water, and she responds nicely to bulge steering and turning strokes. This makes me a minority paddler in New Zealand where most paddlers use a rudder. However if I was to try and paddle 40 miles/day like the legendary Kiwi Paul Caffin I would defiantly add a rudder. It is a personal choice! are you a purist who seeks paddling perfection or are you one who wishes to slip away form the city for a few days of bathing in nature using the boat purely as a mode of transport?