You aren't missing anything. Since there is no MQD rollover and there no bonus benefits for obtaining MQDs beyond 28K MQDs (except maybe a chance at DL 360 invite), you can get a much better ROI by putting that spend elsewhere, possibly looking to maximize points by category with respective cards. The only other consideration is if you haven't hit the mark yet, spending $75K on the Reserve to get unlimited SkyClub access.