When we initially added the runtimes support this was excluded because corefx didn't have any scenarios for it. There are folks that need this. Native binaries can take a dependency on aspects of the {txm}, like WinRT surface area in the case of uap10.x, hosting-APIs/runtimes in the case of net4x.

We've discussed using/mis-using runtimes/{rid}/lib/{txm} for this but that won't work in all cases because the CLI treats native assets differently than lib and any native asset applicable for a core app must be in the native folder.

See https://github.com/dotnet/corefx/issues/8478.