For example, imagine the dependency is "virtual/libc" and somehow you
break the providers of it, i.e. break the glibc recipe. All of a sudden
"bitbake world" would return success even though it built nothing since
anything depending on virtual/libc was magically removed (which is
nearly everything at some point).

The conclusion I came too last time I thought about this was they we
really did want to mark up the cases where things have a specific
requirement, even if at times that is painful.