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.