There are some passenger comfort issues.
Theoretically 3-3-3 offers economy cabin passengers the highest chance of an empty seat next to them. To see why, consider the seat factors that would be required for every passenger to have an empty seat next to them:
3-3-3 = o-o o-o o-o = 6/9 seats occupied = 67%
3-4-2 = o-o o-o- o- = 5/9 seats occupied = 56%
2-5-2 = o- o-o-o o- = 5/9 seats occupied = 56%
2-4-2 = o- o-o- o- = 4/8 seats occupied = 50%
3-4-3 = o-o o-o- o-o = 6/10 seats occupied = 60%
(o = occupied, - = not occupied)
The higher the "empty seat guarantee seat factor", the more likely it is that a passenger will be on such a flight. So 3-3-3 will cause the most passengers possible to have a more comfortable journey.
Now the problem with this approach is that most passengers travel on busy flights and/or that at some stages of the cycle most flights are busy, but that is another story...
