I can envision a situation in which one PlayerData key (or a collection of keys) might contain a large amount of data (for example: saving the state of a 'tycoon' world or other UGC), but other keys might be updated rapidly or in succession (example: world settings). Currently, if the PlayerData gets too big, updating the smaller more frequently updated keys will start to throttle the network.
The docs suggest using PlayerObjects for larger AND more frequent data, but PlayerObjects have an entirely separate API lacking the ability to dynamically specify a key (and currently a separate data limit).
It would be much better if a partial update could be sent over the network so we can update smaller fields without worrying about larger data which is already stored throttling the network.