No Joomla site should be without it! I recently got hacked due to my own errors and after re-installing a backup realized that just having the correct permissions to folders and files is not enough. I bought and installed RSFirewall which not only showed me all the vulnerabilities in my site but enabled my to securely lock and protect it!
If your site does get hacked you can install a backup but you still need to find where and to protect it from further attacks. RSFirewall can save you a lot of time and headaches and the price ... how much do you value your website!