The Mixed Mode debugger in Visual Studio is a completely different debugger than either the native code debugger or managed code debugger. The primary purpose is to tie into both the very complicated managed debugging API and the very complicated native debugging API at the same time to provide a reliable, usable debugger when working with native and managed code simultaneously. Due to how the debuggers are written, the Visual Studio mixed-mode debugging team would have had to completely re-implement edit-and-continue (which happens to be the most technically challenging debugging feature) to include it. I imagine this was cut because:
- It is an extremely challenging feature to re-implement.
- The implementation would cause significant additional coupling to internal changes in the CLR debugging API (and probably the internals) and the native debugging API.
- The performance of the mixed-mode debugger lags the managed code debugger significantly, leading the majority of users to use the latter for general purpose debugging even without considering the ENC feature. As such, implementing the feature would attract a minimal number of users.
The benefits weighted against the practical challenges leads to an overwhelming vote of leaving this feature out.