I wouldn't use plastic pipe all the time, course plenty of people do, but for certain runs its indispensable. It's also often much easier to run in restricted areas than copper, when you don't want joints or there are lots of bends. I particularly like using it where there is a risk of freezing, since the pipe is less likely to be damaged than copper should the unthinkable happen. Copper is pretty easy to uncoil nice and flat. Plastic is much more difficult, it tends to retain it's curves, so at times it can actually be more difficult to run than copper.