I have a lenovo t460p which has 32 gb of this supposedly power hungry RAM and it can make it through a workday on a charge just fine. From what I can tell, the power needs of non-LP RAM are overstated.

AFAIK, the reason ultrabooks have LPDDR3 is because it is very low profile when soldered. If thin is your thing, you want LPDDR3. My t460p has a smaller footprint than a MBP, but it is as thick as a whole stack of them.

That has a 72Wh battery; the Surface Pro 4 has a 40Wh battery, and this is presumably similar. So it's a pretty big difference.

The difference between LPDDR3 and DDR3 isn't dramatic in actual operation; LPDDR3 uses about 70% of the power of DDR3.

It becomes a bigger deal when the machine is in standby, however; LPDDR3 uses 10% the power of DDR3. This would be a big deal for Apple, which also insists on using LPDDR3, and has historically sold machines with standby times measured in weeks or months. Not sure if people have particular expectations of standby times from Surface Pros, but if they're used like tablets you'd want it to be decent. Keeping the memory powered is nearly the only power used when a computer's in standby, so 10x the power usage would be a huge deal.

I have had several windows laptops with lpddr3, and the standby battery life was never that great. Windows just isn't good at managing battery in standby. I think only apple can bring up the power argument for choosing lpddr3 because it only makes a real difference in their case.

Never really used a modern Windows laptop; I remember the whole power management situation being a complete nightmare in 2001 or so, but would have assumed it'd been sorted out by now...