I run MSE, I barely notice it speed wise, and haven't had any malware or viruses.

On your comment: > the proper solution is to have better security on the machine.

Strongly disagree; if you look at this MacDefender issue, there is no flaw in OSX that is causing infection, users are installing it on their own accord. But what anti-malware/virus software can do here is warn or prevent the user from installing this in the first place, as it should detect the virus/malware signature.

This is why you need it even if the underlying OS is secure, to try protect users from themselves.