Jeff, good points, and as some of the comments indicate, a good topic for discussion and clarification.

For the most part whenever I've asked about license and use I've found WordPress vendors to be very transparent. They want you to understand what you are paying for so that expectations are clear. The only time I ever got an evasive reply was from a company whose plugin was not GPL.

Carl's comments about the importance of updates is an important point. While you can install on as many sites are you want, you need to consider if you want to take on the extra responsibility of keeping things updated without the automated convenience or access to the vendor's changes. Some people have the time and attention for that, but many do not.