Master-Slave D-FF vs Edge triggered: timing issues, simulation shoot-through
You are correct that master slave flip-flops provide the nice functionality that you define. 

However there are other situations where you need that first latch on it's own.

For example, where the output of the second latch is dependent on some other input..







simulate this circuit &ndash; Schematic created using CircuitLab

As such you need the simple edge triggered latch as the primitive.