This SO question and answer might help clarify why transformImageColorspace is useful: http://stackoverflow.com/questions/29296779/convert-pdf-with-transparency-and-cmyk-to-jpg
Also, there's a Github issue here that explains the difference between transformImageColorspace and setImageColorspace: https://github.com/rmagick/rmagick/pull/75