Hi :)
For instance, what could be the use of MADictionnaryAccesor ?

If you have a dictionaries with your values ... e.g. a property  
dictionary with different key and value pairs.
Actually, I think I might need one so I m trying to understand how it works.
I'd like to do somethink like that:

    event:= 'avalanche'.
    temperature := #( #value->25  #analyses-> #(1 0) )      
    activity := #(#value->anActivity    #analyses-> #(1 0.8) )

It's a way to add analyses element to an experience (on their attributes)

I'd like to use a MADictionaryAccesor to manage to build description to have a simple editor and viewer "like if" my experience object was:

    event:= 'avalanche'.
    temperature := 25     
    activity := anActivity

but  also, the same one in readonly + editor on analyses element (2 text fields)

I tryed a bit today but I'll look at that deeper tomorow. Is it a case where MADictionaryAccessor could be useful ? (I hope I'm clear enough ;))

Are they always linked to real instance variables (created) ?

No, as Damien said they can be dynamically calculated, fetched from a  
database, dispatched to other objects, etc.

and so they can be seen as extra-information on an object defined in squeak ?

see you :)