One thing that is especially evident in this chapter (though really throughout the entire book), is how unprepared the dwarves are for the actual job they have come for. They were not especially well prepared for the journey east, but they had at least given some thought to it, and bought (in the Shire) a good portion of supplies. Upon arriving at the Lonely Mountain, the place that they have been trying so long to get to, they more or less abandon all thoughts of their plan and just wing it, looking around almost at random for the side entrance and forgetting the inside information that Elrond gave them (moon letters on their map).
This seems to me deeply realistic. I think that it is not terribly common for a group of people to execute according to plan, and much more common to mill about and do whatever comes to mind at the time, regardless of what was said before. Also, thinking about the hard part of a future project is less fun than thinking about the benefits of success. The dwarves have, as far as I can tell, no plan at all for what to do about Smaug, once they find the side entrance.
The one who should be keeping the group together and on plan, and even more at making sure there is a plan, is Thorin. While he does all right in combat, he is not terribly good as a leader. He seems, by disposition and training, to be better suited to the role of sergeant than king.
What, then, to make of Gandalf's decision to send Bilbo along with this hapless group? In one way, it's rather appalling; it could easily have been a death sentence. On the other hand, it is somewhat inspired. Bilbo is both more sensible, cautious, thoughtful (as in using forethought), and perceptive than they are. But, had he sent someone like a Took or perhaps an Elf or Man, they would likely have gotten into an angry argument with the dwarves (Thorin in particular) and stormed off. Bilbo's mild-mannered personality allows him to be listened to, eventually (and just barely).
We should give Thorin credit for that, at least: he does not perceive Bilbo's growing status as "fellow who figures out what to actually do" as a threat to himself. This could be one silver lining of his position as hereditary leader; he cannot be displaced (especially not by a hobbit), so he has no fear of it.
Chapter 12