I tend to agree with Ian that a kit is a starting point for building a boat and that it would be impossible to include all possible variations in the instructions.

Another thing to keep in mind is that one reason some people choose kits is that they have more time than money and it is the least expensive way to get a good performing sea kayak. So price is an important consideration when selecting fittings and accessories.

And if you really don't like to accessories included in a kit, build from plans and source the materials and accessories yourself.

Finally, my personal ranting: Part of being a boatbuilder is dealing with all the little problems that come up during the construction process. Part of the satisfaction of a finished boat is the successful resolution of these problems. There is no better opportunity to put a custom seat in a boat than when you are building it. With a few sketches and some trial and error fitting, most kit builders ought to be able to come up with a seat installation that fits them better than any commercial seat. It's a lot easier to do it with a kit than it is to replace the set in a brand new fiberglass boat.