This is a common debate in the PHP world. Why support backwards compatibility? These are old, buggy versions of PEAR, right? Yes, they are old and buggy versions and anyone using them is asking for trouble, but people may not find a compelling reason to upgrade their PEAR installer just so that they can use your package because what they have already "works for them". It is (or should be) your goal as a package distributor to make the process of upgrading as painless as possible. You should only drop package.xml
version 1.0 support if you are in fact using new features of PEAR itself in the PHP code, or your package is a new one without an installed userbase.
PEAR development has progressed at a rapid pace, but the adoption of the new installer will not happen overnight. Large software projects like Linux distributions need time to evaluate the new features and make sure that everything works properly prior to adopting a new version. As PEAR developers...