Full Text Search is currently the default in the search bar in the Notes Client. But DQL is a great alternative, particularly once it includes non-summary fields / attachments. But there are applications where developers may have set up saved searches.
Once fully feature-compatible with FT search, the search bar in the Notes Clent for new applications should use DQL. For customers who prefer full text search (if any!?), a combo box on the Full Text Search tab of database properties could be used to change it.
Similarly, for pre-existing applications, the same property could be used to switch to DQL.
Within the search bar, the buttons to select fields, operators etc would need changing. But it would be awesome if the search field used typeahead to suggest fields, view columns etc. A checkbox or combobox could be added to choose "In This View" or "In This Database". Displaying the results would be somewhat challenging with the current views because they show View Entries rather than a Document Collection. But I'm sure that would be surmountable.
Of course all of this may already be planned - DQL plans to extend beyond summary fields; I believe changing the search engine from current full-text search was pencilled for V11; better attachment searching is in V10. So aligning all of those is a logical step.