PEX instead of good ol'copper pipe for fresh water in residential house?
The reason this is done is because Copper costs tons more money. Otherwise the 'greatness' of it would not be a challenge to copper as copper has antimicrobial properties and is very durable.

The PEX is run through the house to keep it repairable and simple - I do not think there has been any issue with doing this. 

I have heard they can run it under the slab for in house water supply. 
They do run PEX in the slab for in SLAB heating.

BTW: I like the OLD SCHOOL Copper - works great in the water supply and it works great for Air Conditioning units, and definitely works great for electricity .. next thing you know the Power Lines in the house will be plastic..