This is NOT "official Cakewalk opinion," just my take on the situation as a long-time Sonar user. To do pro-level, engraving-quality notation, it's necessary to have software that's more or less dedicated to that task, like Sibelius or Finale. These cost about the same as X3 Producer. Notation programs have a tiny market share; DAWs, virtual instruments, and effects are about 85% of the music software market. Notation programs don't even show up as a category in most sales surveys, but are lumped with "others" or "misc."
So with respect to Sonar, it's pretty pointless to devote massive amount of resources to providing pro level notation, especially in terms of going against entrenched competition. Those who just want to print out a quick lead sheet can use pretty basic notation which is available in most DAWs, including Sonar.
So, that leaves those who are more "middle of the road" and simply want to be able to interface with a DAW using their preferred method, i.e., notation. They don't need engraving quality, but they want something that is accurate and efficient.
Now we get into areas where I could be wrong, but my understanding is that doing even "middle class"-quality notation is not at all trivial. Also, I believe that in terms of priorities as presented to Cakewalk, decent quality notation falls more into the "vocal minority" category. So if a ton of people want VST3 support, or ARA integration with Melodyne, it's more likely their needs will pop to the top of the stack.
X3d is slated to have numerous video enhancements. If you consider how many people are uploading to YouTube versus those who want to use notation as their primary means of interacting with Sonar, I'm quite sure the numbers favor those who want to upload to YouTube.
Although I rarely need to use notation, I fully understand why some users would want to be able to work primarily with notation, especially if that's how they were raised musically. It's simply faster and more fluid for them. The MusicXML export feature is fine for those who want to print notation, but not useful for interaction.
Of course you can ReWire Notion into Sonar, which some people find an effective workaround for $99. But what I hope is that at some point, a protocol like ReWire and/or ARA will allow a notation "add on" from a third party that's designed to work specifically with DAWs. Then Sonar users could use notation, but Cakewalk wouldn't have to devote the significant amount of resources required to make it possible.