Can someone recap the differences between @serviceGroup vs. definitions-targetNamespace ?

I am a little bit confused by the serviceGroup debate now that we only 
want to group services (possibly with an attribute @serviceGroup) 
without defining more specifically any relationship between them. 
Service elements can already be grouped through the definitions element 
or via the targetNamespace. I am not sure to clearly see the differences 
between these means of grouping and the newly @serviceGroup proposed 
one. Can someone recap why we shouldn't use the definitions element 
(static case)  or targetNamespace (dynamic case) ?
Clealry WSDL allows to separate services/instances elements definitions 
from interfaces/classes definitions.
I believe that in most cases, the owner of the services instances is 
also the owner of the targetNamespace uris and definitions element 
containing those instances. It could be part of a best practice 
guidelines to create definitions elements or targetNamespaces containing 
only services elements if services grouping semantics are needed...

    Youenn