If a library is new enough to not be available as a gem, I'm not sure I'd
want to install it system-wide. You can always put it somewhere in your
home directory and add it to the RUBYLIB environment variable.
I wouldn't be so categorical. There are libraries which are quite new and
still, for various reasons, aren't packaged as gems. One which comes
immediately to my mind, because I use it, is qt-ruby, the ruby bindings for
the Qt toolkit. Other are (I think) ruby-gtk2 and ruby-gnome. Then, there's
fastri, which offers both gem and tar.gz versions whose release note states:
RubyGems adds a noticeable overhead to fri, making it run slower than if you
installed it directly from the tarball with setup.rb.
If someone is concerned about speed, he may choose to install the tar.gz
version.
So, there may be more than one reason for which a library is not released as
gems, although I wouldn't deny that in most cases your statement is correct.
Stefano