Memory protection is just as necessary for security on a multi-user operating system as it is for crash protection. Without it, any application that crashed or decided to be malicious could corrupt just about anything on the system.
Memory protection is also largely implemented in hardware, and is a fundamental component of how multiple processes can coexist on one computer and still appear to run simultaneously.
That's wayyyyyy different from adding hacks to the kernel to fix broken applications. That reduces kernel quality and encourages app developers to be lazy. It's something Microsoft would do -- add kernel hacks to make Office or Borland work right.