The problem is, there may still be bugs in the beta version (even in release version), and if in the beta stage, SUN discovers any bugs, it may change the API, or remove them.
But you may not know if you are writing applications based on beta. If your application is tightly coupled with those APIs, you may find problems when it has been changed, or removed, as the behaviour may then not what you expected at the beginning.
Thus, better wait for offical release for application development purposes.
Nick
SCJP 1.2, OCP 9i DBA, SCWCD 1.3, SCJP 1.4 (SAI), SCJD 1.4, SCWCD 1.4 (Beta), ICED (IBM 287, IBM 484, IBM 486), SCMAD 1.0 (Beta), SCBCD 1.3, ICSD (IBM 288), ICDBA (IBM 700, IBM 701), SCDJWS, ICSD (IBM 348), OCP 10g DBA (Beta), SCJP 5.0 (Beta), SCJA 1.0 (Beta), MCP(70-270), SCBCD 5.0 (Beta), SCJP 6.0, SCEA for JEE5 (in progress)