I was asked off-list why the biblical quotes in the Talmud (and, I will add,
pre-10th century Jewish liturgy and other literature as well) are not used
by scholars as evidence of the "proto-MT". With the permission of the person
who asked, I'll post may answer on-list.
Basically, there are two reasons that I can think of. One is the general
disdain that Christian scholars traditionally have had for all things
Jewish. This is an old and ingrained tradition in Christianity, that
precluded many Christian scholars in the past from even being familiar with
the Talmud, not to mention using it as evidence for anything. By the way,
the animosity was mutual, and it is very rare to find a "traditional" Jewish
source quoting a Christian scholar. Hopefully lists such as this one are
doing their bit to clear the air.
The second reason is a more "serious" one. We don't have any significant
very old mms. of the Talmud either. This means that there will always be the
suspicion, that in cases where the Talmud was quoting what seems to be the
MT (or "proto-MT"), it did originally have a different text, which was
"updated" to fit the GRU, as time went on. How would anyone know?