Do they exist? Do they make you money (or in the case of Firebird meaningfully extend your community)? Would you be unhappy if support would be dropped in PHP 5.3, in PHP 6.0? Do you have ressources to prevent this from happening, by taking ownership of the code in question? Are you interested in ensuring the availability of solid support in PDO? While I do not think support will be dropped in PHP 5.3 (well for FrontBaseSQL I have a hard time standing up in defense like I did for the other 3), there is a good chance this will happen in PHP 6.0. Without PDO support your users will be left more or less in the dust. So if you work for one of these vendors, please talk to who ever who can prevent this. If you know someone at these vendors, please contact them or give me the coordinates so that I can do this for you.

Oh and in case any of you vendors are wondering, the other guys do directly put people on the project: MySQL, Oracle, DB2 (and somewhat Informix), Ingres (yes these guys exist and they have a developer working on the ingres extension). SQLite and PostgreSQL to some extend are supported by the PHP community itself. All the other database extension or PDO drivers are more or less dead in the water in terms of support.

Update 15/06/2006:
This is again a shining example of where we could benefit from a structure like what I proposed to setup as part of emPHPower. This of course also applies to the question of we need to keep the undocumented funky multibyte features that we not yet supported in the re2c based builds of PHP 5.3. With an infrastructure in place to directly ask questions to its user base, PHP core developers could help solve situations like these quickly and for the benefit of the user base.