DrillDown is a useful class with a straight-forward API, which makes the life of basic users simpler.
As Shai pointed out, today there is no dependency on the Queries module, but the code contains a hidden bug in which a 'drill down' operation may change the score of the results. And adding a Filter or a ConstantScoreQuery looks the right way to go.
That sort of a fix is possible, while keeping the usefulness of the DrillDown class, only if the code becomes dependent on the queries module.
On the other hand, removing the dependency would force most faceted users to write that exact extra code as mentioned. Preventing such cases was the reason that utility class was created.
'Drilling Down' is a basic feature of a faceted search application, and the DrillDown class provides an easy way of invoking it.
Having a faceted search application without utilizing the queries module (e.g filtering) seems remote - is there any such scenario?
Module dependency may result with a user loading jars he does not need or care about, but the queries module jar is likely to be found on any faceted search application.
Modules should be independent, but I see enough gain in here. It would not bother me if the faceted module would depend on the query module. I find it logical.
-1 for forcing users to write same code over and over to keep facet module independent of the queries module
+1 for adding DrillDown.filter(CategoryPath...) - That looks like the way to go