Random speculation: Google put this feature in Wave to get some early feedback before including it as the default scrollbar behavior in Chrome OS.

I will most likely be verifiably wrong about this soon, but it would make sense given Google's strategy of testing things out in small batches before making it default. Why the comment about "mobile devices and netbooks"? Why Wave? They could just as well put these scrollbars in Gmail or any other Google product. It doesn't make sense to me, except if they want people to try it out and work out the kinks before putting it in Chrome OS where 1) the primary target is "mobile devices and netbooks" and 2) they can much more easily make that the default scrollbar behavior.