Great question. Hopefully it will attract Jenny Bryant's notice for more insight than I can give.
Looking at use_testthat
function ()
{
check_is_package("use_testthat()")
check_installed("testthat")
use_dependency("testthat", "Suggests")
use_directory(path("tests", "testthat"))
use_template("testthat.R", save_as = path("tests", "testthat.R"),
data = list(name = project_name()))
ui_todo("Call {ui_code('use_test()')} to initialize a basic test file and open it \\\n for editing.")
}
the function was designed solely for use in the context of package development.
But, all that's needed to be a package is a DESCRIPTION file in the working directory, plus whatever canned directory and files that the various usethis functions expect. (But no promise, since I'm not actually, myself, personally, a packager.)
Likewise, with use_mit_license and siblings
{
force(name)
check_is_package("use_mit_license()")
use_description_field("License", "MIT + file LICENSE", overwrite = TRUE)
use_license_template("mit", name)
use_template("license-mit.txt", save_as = "LICENSE", data = license_data(name))
}
yes, it's designed as a packaging component, but should (says the man who's probably not going to get around to trying it first) be relatively easy to adapt.
More from the guy who hasn't gotten around to doing it himself: When I was spending more time in Python I finally got tired enough of tracking down snippets, I made my own util package to import for the good of the cause. On on the next One of These Days, I'm gonna get around to doing the equivalent for an R package. So then .Rprofile can be the bare essentials, util will be the full kit, and specific projects will have either libraries manually attached or namespace attached (through a process I have yet to understand).
Now I'll veer off into the realm of personal choices: 1) should the fabulous here be in .Rprofile, util or as needed? and 2) should gitwork be via pane, package or system? These are intimate questions that must be considered in private between the user and R counselor. 
Thanks for bringing a chance to wax philosophical!