TooL leg curls are very effective at developing the lower part of the hamstrings, where the SLDL doesn't work them very effectively. Remember that the hams are attached in the lower leg, below the knee, and in the lower back, thus spanning two joints, the knee and the hip.
SLDL takes care of hitting the hip extensor function of the hams. Leg curls take care of exercising the knee flexor function of the hams. Doing both in the same workout just about guarantees excellent hamstring development.
A lot of guys spend the first few years training doing only leg curls for hams, and when they discover SLDLs their hams just explode.
I also believe that he full deadlift involves the quads, but in such a sub-maximal way that it is not detrimental to leg development... THIS IS A GENERALIZATION everyone is different of course.