We can probably assume it's not evenly distributed across people. Some people would probably always get cached queries, while others would need fresh results at least half the time.
That's probably true. I imagine it would also be true to say anyone using a new search engine would be those throwing unique queries at it most of the time too.
With that in mind, and with the tech world generally driving adoption of players in the search space I can't see the approach working. Nobody would switch when the queries are massively slower, even if they were 99% accurate.
Google reports 20-25% of queries are unique, way more than I think most of us would expect. http://www.readwriteweb.com/archives/udi_manber_search_is_a_...
We can probably assume it's not evenly distributed across people. Some people would probably always get cached queries, while others would need fresh results at least half the time.