Malicious scripts and blacklists
Besides being frustrating, malware can actually affect your livelihood if you use your website for business. Think about it, if you visit a site once and your computer gets infected, you're not likely to return. You can't blame visitors to your site if they get infected and decide to take future business elsewhere.
Plus, websites that are infected with malware can actually be blacklisted from search engines. Google can detect that your site is overrun with malware and decide to steer people away from your site for their own good.
The result can be ruinous for your reputation and cause your company to be viewed as unreliable and untrustworthy, which is why it's so valuable to have an anti-malware scanner from industry titans like Symantec or GeoTrust.