Click stream is useful, without a doubt. It isn't essential. We had already started the process at Blekko of moving to alternate ways for ranking the index.
That said, if you run the frontend as proposed, you get to collect the clicks. That gives you the click stream you want. If the index returns you a serp with unwrapped links (which it should if it was unbundled from a given search front end) then you could develop analytics around what your particular customers "like" in their links and have a different ranking than perhaps some other front end. One thing that Blekko made really clear for me that the Google idea that there was always the "best" result for that query (aka the I'm Feeling Lucky link) there was often different shades of intent behind the query that aren't part of the query itself. Google felt they could get it in the first 10 links (back before the first 10 links were sponsored content :-)) and often on the page you could see the two or three inferred "intents" (shopping, information, entertainment were common).
That said, if you run the frontend as proposed, you get to collect the clicks. That gives you the click stream you want. If the index returns you a serp with unwrapped links (which it should if it was unbundled from a given search front end) then you could develop analytics around what your particular customers "like" in their links and have a different ranking than perhaps some other front end. One thing that Blekko made really clear for me that the Google idea that there was always the "best" result for that query (aka the I'm Feeling Lucky link) there was often different shades of intent behind the query that aren't part of the query itself. Google felt they could get it in the first 10 links (back before the first 10 links were sponsored content :-)) and often on the page you could see the two or three inferred "intents" (shopping, information, entertainment were common).