I think the geek/nerd girl is somehow more approachable perhaps due to a less "superficial impression". Even though a bombshell can turn me on in a split second and is a greater flight of fantasy, I'm just not that into someone who is overly invested in their looks. Perhaps the geek/nerd girl represents someone who may be more down to earth. Thus, they can still be confident and sexy when they want, however they aren't overly invested in superficial pursuits.
Is this making sense?
I think they represent a greater possibility for a fulfilling long term relationship. Beauty fades over time and the mind makes up the most sexy part of a women.