I'm a bit afraid of infering such rules from experiments, because for
example, i remember adding a default ctor to SVD in the 2.0 branch
because for some user, MSVC failed to generate a default constructor
automatically. Just saying that it's scary because compilers can have
very erratic behavior.
How about instead saying that all that is a good reason to actually do
xpr templates for Quaternion? After Hauke and Gael merge their
branches, the small performance argument should be gone. Also, Gael's
idea for reusable Xpr classes should apply there too, allowing to do
that without writing any new xpr class.