Without knowing the scoring, or if it's total points or H2H, it's really difficult to say much. But, I'm at the keyboard, so...
Since MFL10's was brought up, I took a look at last years's MFL10 D scoring. WIth 28 roster spots in your league, it's obvious that it's not an MFL10, so take this with a grain of salt.
I compared the top 2 D's (Chiefs & Vikings) for the year against 3 different combos 4 & 12 (Eagles & Dolphins), 9 & 10 (Cardinals & Chargers) and 11 & 12 (Buc's & Dolphins). The biggest delta was 4 & 12 over the Vikings by 45 points which could be significant in a winner-take-all like the MFL10's The smallest was 4 & 12 over the Chiefs by 10. Of course it's quite possible last year that somebody could have projected the Cardinals as a top D and picked them as their lone D, but they ended as the 9th scoring D with 47 less than the Chiefs and 20 less than the Vikings.
I didn't look at any of the teams finishing outside of the top 12, but you never know, you might get lucky with the complimentary scoring. Also, picking someone like a Jordan Howard very late instead of a 2nd D could have won the league for someone.
I'll be taking 2 D's, but if someone wants just 1, or 3 or more, no big deal. When I start winning at some significant rate above the norm, maybe I'll feel justified in passing judgement on others. But I won't. As we used to say when I played a lot of online poker, "Don't tap the glass."