You don't need any 3rd party antivirus app to keep your Mac malware-free. Macs are not immune to malware, but no true viruses exist in the wild that can run on Mac OS X, and there never have been any since it was released over 10 years ago. You cannot infect your Mac simply by visiting a website, unzipping a file, opening an email attachment or joining a network. The only malware in the wild that can affect Mac OS X is a handful of trojans, which cannot infect your Mac unless you actively install them, and they can be easily avoided with some basic education, common sense and care in what software you install. Also, Mac OS X Snow Leopard and Lion have anti-malware protection built in, further reducing the need for 3rd party antivirus apps.
The built-in Mac OS X firewall is sufficient. You don't need any Intego products to keep your Mac safe.
Most of those claiming AV is beneficial on a Mac are those trying to sell you an AV product or those who come from a Windows experience who aren't familiar with the malware environment as it relates to Mac OS X. Since no Mac OS X viruses exist and the handful of trojans that do exist are easily avoided by the user not installing them, the only benefit AV has on a Mac is to scan for Windows malware before sending files to a Windows user. Even then, that's not as effective as the Windows user running their own AV, to protect against threats from all sources; not just your Mac. Do yourself a favor and carefully read the link I posted, and you won't have to take my word for it. That post is filled with facts from reliable sources, not opinions.
I wouldn't call it harm, but it can interfere with apps running properly and isn't recommended. Choosing one good firewall is more than sufficient.
As already stated, there ARE no Mac OS X viruses. The only Mac OS X malware that exists in the wild are a few trojans, which require the user to actively install them, usually requiring the user to enter the admin password to do so. Unless the user installs the trojans, they cannot infect your Mac. In addition, Snow Leopard and Lion have built-in detection for those trojans. There is no need for 3rd party antivirus/antimalware software to keep your Mac malware free.
Leave JavaScript enabled, as many sites require it to function properly. There are relatively few sites that will fail to function properly with Java disabled. If you encounter a trusted site that requires it, enable it while you're on that site, then disable it when you leave. That shouldn't happen very often.
Correction: There are zero Mac OS X viruses.
Zero harm to Mac OS X.
None exist for Mac OS X.
Keyloggers require someone with access to install. Cannot be installed without user giving express permission for someone to access.
Not really. Runnning on a standard vs admin account has little or no effect on Mac security. If a user is careless enough to install questionable apps, they'll be equally careless in entering their admin password when prompted.
