I have it from a very reputable source that if one wants to keep the tranny is great shape, the fluid should be changed every 3rd engine oil change (about every 10k). and the filter should be done every 30k. The MB torgue converter is the only one in the business that has a drain plug and its there for a reason. So you should use it. Keeping the fluid fresh also prolongs the life of the seals and O rings and they will last longer. Your tranny will use about 2 gallons of fluid per change at a cost of about $15. Cheap insurance to maintain a complex and costly piece of equipment.
This information comes from a tranny rebuilder in my area www.jie.com.
He only does MB and ZF transmissions. Been in the business for over 20 years. Ill take his advice seriously.