I still use ML without issues. Yes, I'm missing out on new security features, but at least ML got its last security update only several months ago. Heck, I still use Snow Leopard on my MBP.
But it's true ML, and other older OS' are vulnerable to newer attacks. A couple of things we can do to reduce (not prevent all, unfortunately) attacks are:
1. Use a web browser that is supported - Firefox and Chrome. Unfortunately Chrome's support for ML will be ending soon.
2. Keep your apps up to date.