I have no problem with discussing the merits of Petraeus as a viable presidential candidate. But to eliminate him solely because he is a military leader without any further vetting seems arbitrary.
If one does in fact examine history one sees US generals that later became presidents have numbered 12. I am not aware of any shift to military dictatorship during those periods.

List includes:

1. George Washington, Revolutionary War
2. Andrew Jackson, War of 1812
3. William Henry Harrison, War of 1812
4. Zachary Taylor, Mexican War
5. Franklin Pierce, Mexican War
6. Andrew Johnson, Civil War
7. Ulysses Simpson Grant, Civil War
8. Rutherford Birchard Hayes, Civil War
9. James Abram Garfield, Civil War
10. Chester Allan Arthur, Civil War
11. Benjamin Harrison, Civil War
12. Dwight David Eisenhower, World War II