If an attribute expects values consisting of multiple properties than the user is expected to specify all those properties for every write-attribute.
In some cases, when the majority of those properties are changed infrequently but the minority is changed much more often, it is not user friendly to require a user to specify the complete value every time.

There could be a value adjusting mechanism that would fill-in the missing properties of the new values based on those present in the current attribute value.