Plesk regularly releases updates to patch security holes and improve functionality. A cracked license usually breaks the update mechanism to prevent the crack from being detected. This leaves your server running outdated software, making it a sitting duck for known exploits.
If you need to test the full capabilities of Plesk (like the Web Pro or Web Host editions), Plesk offers an official trial. plesk license key free
If you're looking for a free Plesk license key, consider the following: Plesk regularly releases updates to patch security holes