I understand that you can use either a PSR standard to locate files, or tell composer a directory to scan for classes. The documentation recommends using the PSR-4 standard. There is also an option for composer to create an optimized autoloader, which basically generates a full classmap. So why should one use PSR-4 at all if the best way to load is with a classmap?
It makes sense to me to keep the directory structure, since that is a good way to organize anyway. However, it seems like the logical option would be to use PSR-4 loading on development machines, and then classmap for the production environment. That way, you don't have to rebuild your classmap every time you create a new class, but the production environment creates a complete one as a part of the deployment process without an additional call to
./composer.phar dump-autoload -o
Why use a PSR-0 or PSR-4 autoload in composer if classmap is actually faster?
Because it's more practical.
In production, you can use a classmap (with composer dumpautoload -o) because you won't add any new class, but in dev environment it's interesting to have the flexibility provided by PSR-0 or PSR-4 (i.e. nothing to do when adding new classes).
Update: you can also use composer install -o, it's simpler.