Copyright assignment vs Grant of license

[Chris Yoo]

Dear licence-discuss. 

In most, if not all, cases where a dual-licensing scheme is adopted, the
original licensor typically requires a copyright assignment from a
contributor if they wish to licence any contributions made under the open
source licence, under the non-open source licence.

Would it also be possible to achieve the same by inserting into the open
source licence a clause requiring the contributor to to grant an
irrevocable, non-exclusive, royalty-free right to the original licensor to
use any modifications that contributed under any other license of the
original licensors? (Such a clause may be found in s3.b. of the QT Public
License)

My understanding is that in practice, the only difference between a
copyright assignment and a grant of such a license by the contributor, would
be that the original licensor would not be able to enforce a breach of the
copyright in that contribution. Is there are reason that all dual-licensing
(AFAIKO) adopt this rather cumbersome approach of requesting written
copyright assignment? 

Any discussion or assistance on this matter would be much appreciated. 


Regards, 

Chris.