There is one simple reason why you should upgrade: security.
The 1.5 branch of WordPress isn’t getting the security patches that the current development branch is1. And even if someone were vigilantly applying patches to the 1.5 branch, that might not even be enough. Security vulnerabilities are reported to Matt and the team directly. If a vulnerability is not present in the latest version, the person maintaining the 1.5 branch might not hear about an issue until after it’s already in the wild.
I understand why people don’t feel the need to upgrade to 2.0.x when they don’t feel they need the new features added in this release. I don’t necessarily agree with all the choices that were made during the 2.0 development cycle either. I’m not here to play apologist for the active WordPress developers – and they certainly don’t need me to.
While I didn’t need any of the new features/changes/etc. that were added in 2.02, I do need my web sites to be secure.
I trust Matt, Ryan and the rest of the team to be diligent guardians and custodians of WordPress3.