The goal of a Business Analyst:

For the scope of the business being analyzed, coming up with a comprehensive, integrated understanding of essential (i.e., technology independent) functions (processes) and - especially - their essential interrelationships.  Yourdon used to say that this is 98% of an analyst's required work.

This means rigorous as-is modeling, unless, of course, you are one who belives that as-is modeling is not necessary because top management is going to give you a to-be and that your job is just to implement.

Tony