I saw it as advantageous for OPSO and MO. They can keep up with things if they're out due to crew day, etc.Why, though? As a DH, the world was never going to end if someone couldn't get me on email. That's why you have the myriad of other players in your department (or your department is just really small and you're not that important anyway). Also, as a DH and an OIC, a cell phone was much more valuable than email, and that was usually because I was chasing down something in real time (embassy, my CO, husbanding agent, etc). Otherwise I'd go back to my state room and kick the CHENG off the computer and check my email.
I'd argue if you're at a level that you need to be on your email THAT much, you're probably already issued one (CO/XO/CMC/aide/etc)*.
*Note: I'm excluding SELRES here, as I understand not everyone has a computer anymore, so getting your CAC access on an iOS device could be helpful. But for an AD guy...meh.
I agree that it'd be useless for an embarked OIC or junior DH.