<p>I have never quite understood how/why I would use Maven modules (reactor builds).</p>

<p>We have tens of libraries that we share (as dependencies) among our products, and between libraries as well.  If we were to switch to making them Maven modules, how would we set it up, both in SVN and in our working copies?</p>

<p>Do Maven modules really need to be subfolders? Do they need to be subfolders in the SVN repo too?</p>

<p>Assuming you just need subfolders in the working copy, I suppose <code>svn:externals</code> would work to make, say, a "util" library be a module of multiple projects at the same time.  But I've read many bad things about using <code>svn:externals</code> because there is nothing to stop you from modifying the code in the external, but its not tracked.  </p>

<p>Any suggestions?  Am I missing the boat on modules?</p>

No ... a modular project should only be used when the child project is integrated into the parent to create a larger artifact, so an example might be an Enterprise project, where your modules contain the EJB (server and client), the WAR, and then those are combined into an EAR file.  This modularity is only for convenience and can be skipped if preferred.
In the case of reusable libraries, make them independent projects and deploy them to a shared repository.  Then they must be referenced as dependencies in the project using them.