Barlow & Durand (2012) explain how regardless or counselors interviewing skills and reassurances of confidenatility clinets will not always disclose sensitive information that is paramount for a proper diagnosis. Time seems to be the most critical factor continously confirming collected data to validate your initial diagnosis.
You make an excellent point that both tools accomplish the intent of an obtaining enough information to use in developing a preliminary diagnosis. The question becomes, does the clinician wants everything upfront to make