I went a lot of years without having a skeg on any of my kayaks. It is possible to design kayaks so they don't need them. I eventually added a skeg to one kayak because people seemed to expect skegs on sea kayak and I figured I would see what all the fuss was about. I didn't feel it made a world of difference for me. However, they can be used to advantage to give another degree of control, for example dialing in how loose or stiff the tracking is.

In the boats I've had skegs in, I find I rarely need them, and their benefit is variable. Some times, in some conditions, I will find that deploying the skeg will help settle down the kayak, other times, in what seems like very similar conditions, the skeg will make the kayak harder to manage. Instead of saying "quartering seas, steep waves - deploy the skeg", I find the best course of action is to try it and see if it helps. If it does, use it, if it doesn't, don't.

Most people use a skeg to adjust the relative windage balance of the kayak. I.e. adjusting from weathercocking to lee cocking. Most boats will tend to point up into the wind if not corrected (weather cocking). A fully deployed skeg may cause the kayak to point downwind (lee cocking). For this where placing the skeg as far to the stern as practical comes into play. The skeg acts like fletching on an arrow, straightening out the course of the kayak.

That said, most tendency to be turned by the wind (weather or lee cocking) is a result of the kayak side slipping. As the kayak is pushed sideways through the water by the the wind, the bow and stern may present a different amount of drag to the water. The degree to which the grip at the bow and stern are different will result in the kayak turning one way or the other. I.e. the bow and stern are unbalanced and the kayak steers accordingly.

The forward motion through the water of the kayak creates a set of pressures that increase the lateral drag of the boat. A stern mounted skeg works by presenting more lateral drag at the stern of the kayak to balance out this motion-induced locking in of the bow.

Another way to reduce the wind cocking of the kayak is to reduce the side slip. This is where mounting the skeg farther forward comes into play. While a skeg mounted closer to amidships won't have the fletching effect, it can serve to stop the design from being blown sideways as quickly, or sliding sideways down the face of a wave. Because the kayak does not side slip as much it will not wind-cock as much, so on a otherwise well balanced design, a forward mounted skeg should be pretty effective.

Dedicated surf kayaks (like surfboards) have fin placement that often starts even with the seat and a little behind. Thruster fins are placed on either side of the seat and there is often a center fin just behind the cockpit. These are just another form of skeg. The thruster fins help hold the kayak up on the face of the waves, so it does not slip sideways as much.

From what I've seen of what Reg Lake and others have been experimenting with "Skeels" they are playing on steep surfing waves seeking more control. I've been thinking along the same lines for awhile, but have not done anything with my thoughts. I've been contemplating 3 retractable skegs, two mounted into the cheek plate area and one center mounted directly behind the seat, in surf kayak configuration, with the thought that I could get more effective diagonal runs while surfing.

That said, I just yesterday did a 10 mile down wind run in my 14' Petrel Play (wrong boat) without a skeg and only a couple times felt that a skeg would have been helpful.