Advantages of using a RTOS such as QNX or VxWorks instead of Linux?

When developing a solution that requires a real-time operating system, what advantages would an operating system such an QNX or VxWorks have over Linux? Or to put it another way, since these operating system are designed specifically for real-time, embedded use - as opposed to Linux which is a more general system that can be tailored to real-time use - when would you *need* to use one of these operating systems instead of Linux?