It is a very long time promise that is about to be broken. I have been following Maidsafe for more than 3 years and it has always been specified that a SD cost more than an immutable data (price/size ratio is 10 times higher), but in exchange, they can be freely updated, as many times as wanted. Now that SDs are replaced by MDs, this feature should be transferred to them.
It is very useful for sites having a fixed number of dynamic files. You could just replace them, it was easy and free (if their size was < 3KB).