Tankless water heaters?

The more I learn about PEX the more I want to dive in and redo this house!

So if this stuff is so great, saves all the time of soldering, installation and reduces the chance of pipes freezing why the hell do people still use copper and pvc piping in their houses? Just sitting on my couch looking it up on youtube and google I have thought of several things I want to do to the new house that I could accomplish with just about one weekend of crawling around under the house.