derekprior/backbone-on-rails — comment on commit 55eecad5eb2d07e343a8c3af637943e07f9a1156
File:  line 0
By derekprior:

I've seen it elsewhere (for instance, https://github.com/rails/sass-rails/blob/master/sass-rails.gemspec). I believe the thinking is that you may have an app that uses only parts of rails, but not all of it. If you have railties, you have engines, so you can load this gem.