New
#11
Beta testing doesn't really cover security vulnerabilties. These are either discovered during development by various testing tools, or (the vast majority) when the software is released and tested by security researchers, or so called "white hats".
And most of these vulnerabilities take time to discover (months, or years for some)...
Microsoft is probably the company investing the most in security during development, but the OS has become so complex it is inevitable that bugs or insecure features show up. There was around 40 millions lines of code for Windows 7, I doubt it's better for 8 so you can imagine the complexity and opportunity for human errors or oversight there is.
About updates introducing new features and such, this wasn't what the OP was talking about, but specifically Security Updates. If it's not a security patch, feel free not to install it, I very much doubt any other software will break or become incompatible because of that.