Not advertising, but not really DVC...
...I live in a DL hub, so it is relatively rare that another airline has a routing that is convenient enough to make the savings worth it. That tended to make it easier to earn status on DL, which further tipped things in DL's favor. I recently crossed 1MM, which carries permanent DL Gold/SkyPriority Elite, which more or less cements it.
If you fly DL twice or more each year, one of their co-branded AmEx cards is a no-brainer: it pays for itself on waived checked bagage fees, allows you to board early enough to get a spot for your overhead, and provides a 15% discount on miles redemptions. Per TPG valuations, DL miles aren't great at 1.2 cents/mile. But with the 15% redemption discount, they get bumped up somewhere between "not too bad" and "pretty good". The last flight I booked with miles was 1.38 c/mi, which is more or less right in line with that.
Maybe the bigger problem is that DL is often on the more expensive end for a trip--and sometimes by a lot. In exchange the DL flight is usually non-stop vs. a competitor which has a connection. I'm at a point in my life where I am willing to pay for that convenience.