Dust: The aim is allow 12 OR 24V operation. The reg is to allow the Due to drive the FETs properly, as well as powering the Due (I don't think the Due can handle >12V input to run it). We could use 5V for the FET gate drive, but it won't give you the best performance, so the FETs will get warmer than they need to.
One advantage of using a reg is that the user can populate it with whatever reg they choose. eg: 9V reg for 12V operation, 12V or 15V reg for 24V operation, or if they want to be cheap, leave it off all-together (as long as they're below 20V input).
The main advantage of using a 12V/15V reg for 24V (instead of a 9V) is that by dropping less volts, we reduce how much heat is has to be dissipated by the reg. Even with small currents, dropping 15V on a standard reg produces a lot of heat.
If the only thing I have to change to use a board at 24V is a single reg, then I'm quite happy. We'll need to document it heavily of course, but it's a small price to pay.
Note: You probably want a diode on either the reg input or output if the output can get voltage from somewhere else. I know that Mega's will happily supply voltage back out their VIN line when powered via USB. Many 3-pin regs also do this (any voltage on the input makes it to the reg's output, unless the output is already a higher voltage). I saw this happen when someone replaced the RAMPS diode D1 with a 9V reg on their setup (they wanted to use 15V, and the Arduino wouldn't handle that by default), then plugged the board in via USB only. The small 12V fan that was across the supply output (to cool the board) was slowly ticking over, which is what brought it to my attention.
One advantage of using a reg is that the user can populate it with whatever reg they choose. eg: 9V reg for 12V operation, 12V or 15V reg for 24V operation, or if they want to be cheap, leave it off all-together (as long as they're below 20V input).
The main advantage of using a 12V/15V reg for 24V (instead of a 9V) is that by dropping less volts, we reduce how much heat is has to be dissipated by the reg. Even with small currents, dropping 15V on a standard reg produces a lot of heat.
If the only thing I have to change to use a board at 24V is a single reg, then I'm quite happy. We'll need to document it heavily of course, but it's a small price to pay.
Note: You probably want a diode on either the reg input or output if the output can get voltage from somewhere else. I know that Mega's will happily supply voltage back out their VIN line when powered via USB. Many 3-pin regs also do this (any voltage on the input makes it to the reg's output, unless the output is already a higher voltage). I saw this happen when someone replaced the RAMPS diode D1 with a 9V reg on their setup (they wanted to use 15V, and the Arduino wouldn't handle that by default), then plugged the board in via USB only. The small 12V fan that was across the supply output (to cool the board) was slowly ticking over, which is what brought it to my attention.