Berry, biscuit joinery actually is a mortise and floating tenon joint. The reason conventional mortise and tenon joints are prefered for door frames is strength. When building the doors you want as much material as possible involved in the joint to reduce the likelihood of the door sagging or joint failure. It helps to think of wood as a bundle of straws held tightly together. Glue adheres well to grains running in the same direction or at right angles to each other like in plywood. End grain does not offer enough purchase for the glue to hold. Anything we do to increase the amount of glue surfaces makes the joint stronger. A butt joint has one glue surface. Changing this to a rabbited joint gives 3 glue surfaces. A half lap joint also offers 3 glue surfaces but with a much larger area hence stronger. A through tenon offers 4 surfaces and a regular mortise and tenon offers 6 glue surfaces. This is the reason you see stacked biscuit joints for added strength, 7 glue surfaces but a much smaller area. Needless to say, solid wood is stronger than glueing a biscuit across the grain, and this is why conventional mortise and tenon is preferred..
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Mike
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