This may make sense for your H&H, BMP testing, but requires manual centrifugation, aliquoting, or “front-loading” by the laboratory scientist. There is simply not enough scientists to do this on every patient in the hospital. Your PT/PTT must have full blue for accurate results. https://twitter.com/bryanboling/status/1289504188934664193

Patients with antibodies require extensive work ups in the blood bank, which requires lots of plasma: a full pink top is ideal for a Type and Screen.

If any of your chemistry tests need to be diluted to produce an accurate result (because the analyzers do have a limit of linearity), an insufficient sample will lead to the patient getting redrawn.

As my other colleagues have also pointed out on this thread, analyzers can sometimes missample, due to a clot in the sample, bubbles, etc. In a full tube we are able to rerun the test. In a microtainer tube, we are not.

Also throw any investigating what appears to be erroneous results. Scientists are trained to identify issues with results, investigate them (often retesting the sample), and verifying them before they are delivered to the patient care staff.