I would imagine the clutch is often left out because not only does it save weight to leave it out, but you don't have to figure out how to attach the flywheel to your electric motor, and the flywheel is used as part of the clutch (it's one half of the pressure plate) it also allows for a simpler flex coupler (lovejoy coupling)
so you are simply connecting one shaft to another shaft, and have some leeway for some misalignment... altho you should always make sure shafts line up as perfectly as possible! but if you can figure out a way to get your motor shaft perfectly aline to your tranny, and attach the fly wheel to it, then go for it.
how a car clutch works is: you have 3 layers, attached to the engine is the fly wheel with a smooth steel surface, then you have a double sided clutch disk/friction disk of stuff simaler to brake pads that will "grab" the smooth metal on the flywheel surface that it's pressed agenst, then you have another pressure pressure plate that is simaler to the steel to the fly wheel, that is held to the fly wheel with a weird type of spring fingers, this sandwich squeeze the middle friction layer between the two smooth steel layers, simaler to if you were to pinch either side of a spinning disk your hand would want to travel around with the disk, when you press the clutch pedal it takes the tension off the springs and lets go of the center disk, that center disk is attached to the transmission shaft.
