Based on customer feedback:

At times customers need both CDN enabled and combined styles. Since it's impractical to have all possible combinations ready on the CDN, a per skin combination sounds more reasonable. I.e. Default.all.css or something similar.

That's for CDN.

When running StyleSheetManager locally, again bundling (or defining groups) is a nice addition, as it allows a more specific manner.

Of course such explicit feature, should and must negate the effect of counting selectors and splitting files.