The paid option hasn't been explored yet, and for good reason I think: in principle, you need training data for it to be any good. And, again in principle, the only way to amass user data is for the service to be free, leveraging that to sharpen the tool.
So in principle, I reckon this is doomed to fail. But I might be wrong. I HOPE I'm wrong. And that's enough.
I happen to disagree; almost any for-profit business is going to be doing some sort of aggregated usage data. I mean at the most basic level they've got to be tracking the number of customers they have. That doesn't mean all for-profit businesses ultimately devolve into data selling businesses.
Although perhaps ohduran is advancing a more nuanced argument. In particular perhaps the more detailed usage data you track, the more likely the siren call of selling that data is to be attractive. In order to compete with Google on search quality, perhaps you do need sufficiently detailed usage data that the call becomes irresistible.
I'm still not convinced that's true, but I could see how it plays out.
In that regard, the "siren call", as dwohnitmok says, it's a very appropriate way of encapsulating what I meant. You can be bold and not do it, but as soon as you have investors, they are going to demand it , pressure you into doing it, and if you do not comply replace you with someone who will not be sitting in a potentially profitable line of business and do nothing.
I don't see how a paid search engine has a disadvantage here.
So in practice, the more data you have, the better the engine is. I don't have a theoretical reason for why that is the case, but thing is I don't actually need it.