When we register as a Mac developer, which is free, we automatically get access to some great resources, which include the following:
Access to Development Videos, including World Wide Developer Conferences sessions.
Access to the Mac OS X Developer Library.
Access to the Apple bug reporter system.
Access to pre-release (beta) software.
The opportunity to join a paid developer program.
It only makes sense to join a paid developer program if the benefits out weigh the costs (which can be as low as $99/year). The benefits we get by joining the Mac developer program are as follows:
Access to developer forums
Code level technical support
Access to iCloud services
A developer ID for Gatekeeper
The ability to distribute our App through the Mac App store