Earlier quoted context omitted.
Yes, it would be useful. When controlling for variables, you normally want to compare against a baseline. If 40% of the whole population has THC in them, we'd need a control population (maybe from earlier when thc was less prominent) to see if per capita deaths has meaningfully increased. I'd do the same study, tangentially, for tech workers to see if productivity has changed when controlling for other variables.
No it wouldn't. That would be true if you looked at a variable which is not influenced by driving, like the percentage that wear red jumpers, but one would hope that not everyone is reckless enough to be highly intoxicated and drive. This is again THC apologizism, nobody would even begin to suggest this if we were talking about alcohol.
When we talk about alcohol, we explicitly separate presence from impairment using blood alcohol concentration. We set legal thresholds because studies show a sharp increase in crash risk above those levels, relative to sober drivers. If alcohol were evaluated by merely asking "was alcohol present?" we would massively overestimate its causal role the same way THC is being overestimated here.
The problem with THC data is not that baseline comparisons are illegitimate; it's that we lack an agreed-upon, time-linked impairment metric comparable to BAC. THC metabolites persist long after intoxication, so presence alone is a weak proxy for risk.
So applying baseline controls to THC is not "apologism", it's applying the same evidentiary standards we already demand for alcohol, so the opposite of what you said.