No, you do not need a premium account to play and may join using a cracked launcher. Premium players can enable the auto-login feature so logging on is quick and simple. To enable this, you must have a registered account on the server and have a premium account [An paid account bought from the official Minecraft website].
Once registration is complete, a GUI will open and you can simply click the respective blocks labelled
Cracked. If this GUI does not open, try re-logging.