I think what is important that we don't happen to depend on libraries that would not be accepted in Phobos, like Tango or some port of the Java standard library, ...
Then when striving for an inclusion in the core distribution, all non dub specific functionality should become part of phobos. In the long run, even the dub library itself could be part of phobos. But I think it is a good idea not to rush into things, because real world use before inclusion in phobos is always a good thing.
If non dub specific functionality would be accepted only if it was part of dub's source code, I think this would just be cheating without taking into account the original motivation for the restriction that we should only depend on phobos.
Do you know any guidelines/restrictions on what libraries can become part of phobos or not?