Do you have examples of packages that benefit substantially from byte-compilation?
I would prefer to avoid byte-compiling files when it is not necessary, in particular when installing hundreds of packages, since it takes significantly longer to byte-compile. A comment in the package.el source even suggests that "Compilation should be done as a separate, optional, step."
The Byte Compiling section of the Emacs manual states: "As a practical matter, for most things you might do to customize or extend Emacs, you do not need to byte compile".
The Elisp manual includes an example of the performance of byte-compiled code: "In this example, the interpreted code required 10 seconds to run, whereas the byte-compiled code required less than 4 seconds. These results are representative, but actual results may vary."
EDIT: Where "substantially" means a measurable difference like the example above in the running time of a package feature, or a measure of the latency of UI interaction.
cl) package will greatly benefit from bytecompiling it. In general, anything that has many macros in it will benefit a lot. For some code the behavior is different, or only available if compiling (say, if some code is wrapped in
when-compilemacro and similar.