When someone purchases software, they are buying a license to use the software and receive software updates and support for a specified period.
Some web developers purchase software and hold the licenses on behalf of their clients. Other developers (Buzzquake is in this camp) believe customers should own the software used to build their websites. When you own the software you have total control over it — when to update it, etc. When the developer owns it, the customer is dependent on that developer for license renewal and software updates.
In some cases, our clients have had to repurchase software that was previously purchased by a developer on their behalf when the client and developer parted ways and the developer was unwilling to change the software license over to client ownership.
In summary, we recommend that website owners purchase the software for their websites.