When restarting an unfinished install, the 'Verify requirements' task is skipped and the installer directly moves on to 'Setup database' or a later step.
In the meantime however, the requirements might have changed or they might not be met anymore.
The code base might have been updated, php.ini might have been changed, etc. So we can't be sure anymore if the install meets the requirements.
In my case here, 'drush make' replaced the 'files' directory with an invalid symbolic link. Still the installer moved on without a writable filesystem which caused warnings in later tasks.
So we should make sure that the 'Verify requirements' task is never skipped. This should come without any costs, because if all requirements are met, the requirements page doesn't show up, and the install continues just as it does now.