Answer from cs61c-ar (Albert Chung 15826273) for Question 3
The benefits of pipelining come from instruction thoroughput, being able to operate multiple instructions at once.  Thus, the processor must be able to load new instructions from the memory at the same time as operating on operands for a different instruction from the data memory.  By combining the 2 memories into 1, we no longer have this ability to multitask.