Yes some more recent models, predominantly made for racing do have moulded rails along the bottom.
Two reasons, one to add strength, especially if of thin material and secondly to provide lateral stability at the high speeds encountered.

When the Aerokits were designed in around 1960s plastic was not commonly available to hobbyists and models were designed using wood/plywood and the originals did not even have spray rails fitted. The running gear was also heavy and bulky resulting in much heavier models than are possible today so the hulls did not plane so easily and the rails were not needed for most models.

Technology can now turn out hulls in bulk using extruded and formed plastic and the rails add to the strength to help keep the shape. You can see this on most plastic packaging used for consumer items

There are some fibreglass boats have them built in in fibreglass along the under side of the hull and not the side edge. Why is this. I don't understand this.

My Sea Commander "Jaydee" has spray rails about 3/16th inch square. Have a look at the Aerokits photo gallery. There is a picture of it out of the water and some in action. I believe rails are a must for stability in the turns when going at speed as well as helping lift the boat onto plane.
Ian

Yes you do need to fit.
Can't remember the exact size but about 1/8" square should suffice. I fixed mine with epoxy and cocktail sticks pushed thro a hole drilled in the rail and hull. Make sure you seal well once finished as any water will be under pressure at speed and will find the smallest of holes!
The pic of mine shows the rail just above the joint on the side skins rising slightly at the front. This was an after fit as the water was filling the cockpit at speed.
Position is not too important but my model is light (brushless + LiPo) and sits high in the water. if your model floats lower you may need to fit slightly higher, its really the bit that's out of the water when planing that needs to deflect the spray.