By (re)enabling the Java plugin, have a compromised my security or malware vulnerability?
If you leave
the plugin enabled, you are indeed at increased risk for malware that exploits Java vulnerabilities (assuming you cannot or have not updated the plugin and/or Java to benefit from any vulnerability fixes).
That said, however, you can always temporarily
enable your browser's Java plugin whenever you are sure that the web site you visit is secure and not compromised itself. Likewise, you can temporarily enable Java to run local Java-based apps on your Mac.