I've only ever seen rudders on divorce kayaks (otherwise known as tandem kayaks).
On the other hand, a rudder could be useful if your stroke is much harder on one side than the other -- or if you've only got one arm.
When the industry first started going mainstream in North America, most everything was longer touring models and rudders were the norm. I suspect builders knew that most of their clientele were new to the sport and most purchases were being made by outfitters and touring companies who were trying to make it as easy as possible for people to get out there. Now, a skeg is certainly more the norm and some have nothing at all.
Pretty much everyone has a stronger stroke on one side or the other. If the look of keeping your strokes evenly timed on each side is important to you (it does look nicer,) then the proper technique is to use a bit more of an outward sweep on one side to balance things up. This is also the best way to negate the effects of a cross wind without losing your pace.
I'm actually not one of those old crusty rubber-booters who frowns on a rudder though. Whatever it takes to get you out there and enjoying yourself is a good thing.
My first boat, (that I could never bring myself to sell or trade)is a 1995 Seaward Quest and it actually has a rudder. However, I never use it. I keep it bungeed down because it's older style mechanism was bad to fling the rudder out if you had to roll. Trying to roll up with this big fin fighting against you is very tough. I keep it in working order though for guests and surprise hurricanes.
