as someone who works in a genome center, and also with plants in the past, it is convenient to require all samples be the same. this way you have a smaller set of problems to troubleshoot when things dont' work. also, when results are much more predictable, scorable, all reducing the time to return results. this by the way is always a problem in DNA type situations. at some point, every sample has to be standardized, and the earlier the better, particularly if it is before the center receives it. this way, you only need one pipeline, one set of protocols, less legacy knowledge, greater ease of transfer of knowledge, overall lower costs. also, the blood sample allows one to forward some of that on for more bloodwork for genoypting for ET requirements. one could also use the sample for a future test like monkey mouth, etc.
when one must use different types of samples, it rarely is streamlinable, so it is much easier from the "bigger" picture, to force (force?) the collector to conform to a protocol that is easiest for the lab to process cheaply, quickly, uniformly.
monkey mouth, when for monkey mouth????????????????