Just FYI Fish,
In it's very first organized year, the D2 cars averaged 22 at Lincoln and 20 at Jacksonville. The high count being 29 at Lincoln, and the low of 12 also at Lincoln.
To put some perspective on that: Lincoln's average Modified count was 16, and their Pro Late Model count was 15.
I don't know IMRA's totals other than they always drew well at Spoon River, enough in fact for Spoon River to bump up the D2 purse for this year.... I think their highest car count was 31??

Now in Indiana it's been a bit slower, but looks like it will pick up some this year. With that said, D2 cars at Montpelier consistently were half, and sometimes more than half of the field.
In response to the initial post, i think most (read that most, not all.... NEMA), that get into Midget racing, are not into running a wing. If they were, there are plenty of other options out there to do so.

The Waynesfield group allows D2 when they run non wing, and I hope that MMSA in Indiana would someday do the same. Andy's group (MMSA) is very strong as it is, but I doubt promoters would be horribly upset with the spike in car count at the non wing shows.