Many projects using Subversion avoid svn:externals and instead use sparse directories that came with svn 1.5

svn co --depth=files http://svn.example.com/software/trunk
svn update trunk/my_project trunk/shared

This way developers get a smaller working set of files but one that behaves correctly.

Generally, more advanced options for how a set of files is monitored and checked out would be very helpful.