"...There aren't any real benefits to a "finger prick" other than marketing spin..."

"...regular draws but used the best trained phlebotomists who can find the vein on the first stick..."

That's the advantage of a finger prick. You don't need to be a trained phlebotomist to do it. The microfluidics drove the idea that you only needed "a few drops" to perform the tests (since drawing 100mL out of a finger would take forever). Put those two together and you had the concept of a test that any semi-skilled person could perform in a drugstore. That was the hook Theranos was banking on.