Say there's a hole in your web browser that lets an attacker run arbitrary code. That's bad news for your home directory, but at least if the binaries on your machine (under /usr/local or wherever) are owned by root, the browser is running as "you" so it can't overwrite them with malware.<p>Then again, making /usr/local writable by admins goes along with the OS X convention of giving admins write permission to /Applications but I'm not a fan of that either.