If you deliberately ignore the fact that 'R CMD INSTALL' is also used
by developers to install from the *package source tree* (by opposition
to end users who use it to install from a *source tarball*,
.. for a good reason, IMHO no serious developer would do that for obvious reasons - you'd be working on a dirty copy creating many unnecessary problems and polluting your sources. The first time you'll spend an hour chasing a non-existent problem due to stale binary objects in your tree you'll learn that lesson ;). The fraction of a second spent in R CMD build is well worth the hours saved. IMHO the only valid reason to run INSTALL on a (freshly unpacked tar ball) directory is to capture config.log.