Hey, @ericaVoss, Thanks for talking this over with me offline.
My misunderstanding was how we see measurement values come across in the real world vs. a theoretical situation where a measurement of a particular type always being recorded as a number (not categorical) and thus there would be no value to map over to a categorical concept (stored in ‘value_as_concept’) so I was arguing the use of NULL in that case. But, from an analytical perspective, storing value_as_concept_id = 0 is no different from storing a null because there’s not a analytical case where you’d want to process unknown/missing values.
I agree that we don’t want to get hung up in the various flavors of ‘null’. When we see a 0 in the value_as_concept_id, we’ll be able to refer to the ‘value_source_value’ to understand what the original value was from the source. And to make this the convention for all *_CONCEPT fields, feels awkward, but I can’t argue that there is a critical problem with that approach that will cause system failure.
I’m drawing the line, tho when we make 000-00-00 the ‘unknown or missing date’ value! 
-Chris