I've been wondering the very same thing about Vit B1 and B6 specifically. What about the total picture rather, than an ad hoc look at B12 and Vit D, which can be very equivocal anyway.

Is it even worth asking for these things to be looked at?

Also what about the phospholipids (such as phosphatidylserine and phosphatidylcholine etc) - how complicated are they to test for? And what are their implications?