A merger is when two or more entities become unified into one entity
like two companies becoming one single company. So, /usr merging
should require other directories becoming part of it. Googling brought
me a question on Ubuntu forums which asked: "Are {/bin, /lib, /sbin}
symlinks into /usr in Ubuntu?" If that is what is understood by this
'fearsome' unification then /bin, /lib and /sbin should become
symlinks with their usual contents moved to /usr. A program should
still be able to find files under /bin, /lib and /sbin provided there
are no two different files with the same name. A disadvantage of the
merger is a merged /usr requires more storage space than before. This
should not be a problem with modern hardware that is more energy
efficient, quieter, and smaller in physical size. Finally, a merger of
a system base directory should not bring instability problems like the
changing of an insufficiently debugged system core executable.

Problems may, however, arise with setups that specifically require
/usr to be unmerged. Here, the question should be how frequent these
setups are, and what role they play in setups that do not tolerate
downtime.