I'm a big fan of questioning our assumptions, so thanks for this conversation.
"The primary purpose of SCD start/end dates is not for end user querying, it is for the purposes of doing lookups during the ETL process."
Right, not end-user querying (is it common to expose dimension tables to end-users?) but what about the ETL Queries? They need to be fast and accurate.
In our data, most of our data is not contiguous. We need to know when there is no value for a member, not just which value applies at what time. I don't see how that's possible without end_dates.
