[dmm]

Slightly off topic:
It seems to me that most solar systems should have LOTS of tiny no-atmosphere or thin-atmosphere planets, especially around the fringes. In other words, a Kuiper belt.

Also, if you're going to aim for realism, then planets must have temperature attributes. I would think that the extremes of temperature would be more important for colonization than atmosphere type. If a planet's climate at its equator is like Antarctica in winter, then you're not going to get undomed colonies of humans even if the atmosphere is nitrogen/oxygen. Conversely, if a planet is very Earth-like except that it has little atmospheric oxygen, then colonists just need to carry an O2 supply/generator when they go outside -- domes aren't really needed.