Look, there's a simple and correct rationale for making the switch a mute switch.
There is NEVER any reason to lock the orientation if the screen is off. Never. Not a single justifiable use case.
There IS a reason to mute the sound with the screen off (since the device can make sounds even when the screen is off). You may choose to blame the victim for "forgetting" to mute the device, but the simple matter is that the number of people who find themselves in situations where they need to quickly mute the sound with the screen off is X. The number of people who find themselves in situations where they need to quickly lock the orientation when the screen is off is 0.
And X>0.
FFS, how often does it need to be said? YOU CAN ENGAGE THE MUTE BY HOLDING DOWN THE VOLUME BUTTON FOR 1.5 SECONDS, EVEN WHEN THE SCREEN IS LOCKED, THEREFORE HAVING A MUTE SWITCH IS REDUNDANT AS YOU WILL STILL BE ABLE TO MUTE USING THE VOLUME ROCKER IN 4.2, GIVING YOU TWO HARDWARE SWITCHES FOR MUTE.
Replacing the rotation lock switch with a 4 step process is inefficient no matter how you try and sell it as it comes in handy when reading whilst lying on you back, as the iPad continually flips rotation when held parallel to the ground, or when handing it to someone unfamiliar with the auto-rotation feature, which also happens very often.
Could we please restrict responses to this issue to those who actually have and use an iPad? Because if you haven't used one, then you do not posess the exeperience to make informed and rational comments.