This is out of date. It's been years and years since GUCs required Y B M fares on DL operated flights.

Personally, I don't see the value of accumulating a stash of rollover MQMs since DL could eliminate them at any point or up the MQD requirements to make them worthless (which has already happened somewhat with DL's decision to raise the AmEx MQD waiver for DM to $250,000).