I have thought about it a bit. One method is optimizing for session success rather than query success. That rewards interfaces that appropriately surface tools that help searchers along in their journeys. Another is to model both the engagement with the tools (on a per-query level) and the performance of the resulting query. Again, that encourages surfacing tools when they are appropriate and not only likely to attract user engagement but also to reward that engagement. And of course a big part of the story is query specificity.