Today, when expansions are defined for a collection, the expansions are always applied. However, there are some cases where one would like to submit a query without having it expanded, like a "do not expand" flag.
A concrete case is the following: When building a tool to actually generate an expansion dictionary, merging synonym proposals from existing thesauri, it would be useful to determine whether a proposed synonym is actually being mentioned in the collection. If it is not mentioned at all, that synonym would get a lower weight. But once the first version of the query expansion dictionary has been deployed, and constantly new synonym proposals are processed, there might already be an expansion defined for a proposed synonym, so WDS would return documents although that synonym might not even be mentioned in any document. This would screw up the weight attributed to that synonym.
Another case would be when trying to evaluate the positive effect of the query expansion using the same collection. It is just not possible today.
And there could be other cases where one might just want to search for that literal term on a per-query basis.