DHH seems fairly convinced that CS "lifts JS/CSS handling in Rails to a whole new level. Very excited to have this as part of Rails". So it would seem the point

DHH seems fairly convinced that CS "lifts JS/CSS handling in Rails to a whole new level. Very excited to have this as part of Rails". So it would seem the point of making it default is to make it so one can't think of Rails and not CoffeeScript. I don't really have an opinion either way as my JS skills are minimal and I know almost nothing about CS.