Since Piwik does NOT follow PSR-0, composer makes unnecessary file system calls for every single class. Piwik's own autoloader knows how to load Piwik's classes.
Can you please point out where Piwik does not comply with PSR-0? The main goal right now is to be compatible to PSR-0 rather than removing the statement.
According to PSR-0, namespace is translated to directory structure by changing backslashes to directory separators. When composer is trying to load a class, lets take \Piwik\Plugins\Actions\API for example, it checks if a file plugins/Piwik/Plugins/Actions/API.php exists. Since it does not, PHP asks Piwik's autoloader (which is second in the autoloader stack) if it can find the class.
I already commented in http://dev.piwik.org/trac/ticket/4074#comment:2 about moving the directories to comply with PSR-0.