What your forgetting, Cali, is that in a DoN you don't need to get first place to get $6 - you only have to survive to 5th.  If you want to get real technical and number-y (I know you love numbers ), here's the basic stats - assuming every player has equal skill level:

Odds of winning a $3 DoN: 1/2 or 50%
Expected value of a $3 DoN: ($6)*.50 = $3

Odds of taking first in a regular $3 SnG: 1/9 or 11.1%
Expected value, 1st place: ($15)*.111 = $1.66667
Odds of taking second in a regular SnG: 1/9 or 11.1%
Expected value, 2nd place: ($9)*.111 = $1
Odds of taking third in a regular SnG: 1/9 or 11.1%
Expected value, 3rd place: ($6)*.111 = $0.33333
Total expected value, regular SnG: $1.67+$1+$0.33 = $3

Your expected value in either game is the same - your buy-in, minus the rake.

You are absolutely correct.  Standard 9-10 handed SnGs are more +ev for those who have a specialty in short-handed play.  Meanwhile, the DoNs are great for those who are pretty decent players, but aren't able to make the transition from full handed all the way down to heads up.

The DoNs are great practice for players who are relatively new to the game, and for those who want to practice their mid-games in MTTs (where blinds start getting a bit high, and no one wants to bust).  Meanwhile, the SnGs are very good practice for final tables (although in an MTT, you'll most likely reach the final table with a decent read on each player - whereas in the SnG, you have no reads).

35:15