After I began using PDFCompress for minimizing PDF files, the only reason I have had for using the full Adobe Acrobat has been to combine PDFs. Now I realize that since OS 10.5 this functionality has been built into Preview. I guess I should really start reading the release notes of OSes and applications a bit more carefully, since I managed to get to 10.6 before I found out about this feature.
Anyways, it is super easy to combine files: Just open in Preview and drag the icon(s) from one file on top of the icon in another (see demonstration).
Looking around for this solution I also discovered the free iCombiner application, which may be even easier to work with, and especially when having many PDF files that I want to combine (e.g. lots of chapters in a book).
So now I don’t really see any good reasons for upgrading my old Acrobat to a newer version any longer.