I think your question requires more context: who are your users, and what kinds of information do you expect them to be looking for? Is this expectation based on pilot testing or other user-centered analysis, or is it something the system designers anticipate without having specific evidence one way or the other?
While it's common wisdom that people don't use long queries, there are many situations where that isn't the case. You need to define some use cases, scenarios, personas, or whatever that will indicate the need for advanced search. If you have a query log from a running system, look for sequences of queries from the same person: chances are these indicate that at least some people have information needs that cannot be articulated in a couple of terms.
Once you have some evidence of the kinds of complex information needs (if any) people have, you can better articulate your original question.