Thanks for the responses. The picture is becoming clearer. Seems to be a number of factors:
1. Aircraft structural considerations
2. Aircraft operational considerations
3. Aircraft performance considerations
4. Operator financial considerations
I guess the last one is really a bit of a trick. For those of you flying aircraft with 'artificially' lowered MLW: What would you do if you had to land above your stated MLW but below the aircraft's actual design MLW. Would you follow the overweight landing checklist? I'm sure you would log the event, just to satisfy formalities.