If you used the plain.tex versions of matrix, pmatrix, or
cases in a document and then later converted the document to use the
amsmath package (or one of the AMS documentclasses, which
automatically call the amsmath package internally), the instances
of those commands will produce error messages. The problem
is that when was originally created, it adopted most of its
mathematics features straight from plain.tex. But in the case of
matrix, pmatrix, cases this was a mistake—the plain.tex
syntax for them is decidedly non- in style, for example the fact
that they use cr instead of
to mark line breaks, and they
don't use begin and end. In basic this mistake will be
perpetuated at least until 3 appears, in order to avoid breaking
existing documents. But no existing documents that were written with the
amsmath package have that syntactic problem, as amsmath
provides proper -syntax versions of matrix and the others.
The possibility of optionally allowing the plain.tex variants to
make document conversion easier seems ill-advised since those variants
are so blatantly wrong in a context.