MySQLi is a “new”1 and improved version of the original MySQL extension for PHP. While there is no especially convincing reason to switch from the original MySQL functions to MySQLi, both MySQL and PHP recommend that developers do so.
WordPress is aimed for the masses and WordPress 2.0 still supports even MySQL 3.x – so most likely we won’t be seeing a switch to a MySQLi-powered WordPress anytime soon. However, for those that care about such things, we’ve created a MySQLi conversion file for WordPress.
It’s been available for years, just no one uses it. ↩