In Rails 3.1, Sprockets are used to manage assets and package them into a single file. Which, in general, is not such a bad idea.
As quoted from an external source, which explains the issue at hand:
A problem with this approach is that it could make debugging harder, if you have to look at the "concatenated" CSS file in production to make sense of what code's included and not, it's harder to know what comes from where than if you just included the original source code files.
One solution would be to have a way to switch between "concatenated" and "normal" modes easily (maybe it's already possible, I don't know), so that normal development would be unimpeded. But you'd have to resort to the big concatenated file for debugging in production.
In Rails 3.0.X, our designer could easily pin-point the CSS setting using Firebug, which will indicate the file and line number directly, since all CSS files were separate and not packaged into one.
Or am I missing the point?