Now that I'm thinking about it some more, one of the motivating use
cases for extras in entry points was startup performance in
plugin-heavy GUI applications like Chandler. The use of extras allows
for late-loading of additions to sys.path. IOW, it's intended more
for a situation where not only are the entry points imported late, but
you also want as few plugins as possible on sys.path to start with, in
order to have fast startup.