Bilbo is the burglar of the group, and it is his job to go and investigate the scene.
In regards to the central themes: heroism, wisdom and nobility we can add the complexities of a noble thief: is this an oxymoron? And in terms of heroes it is ironic that the dwarves were sacked after rather timidly relegating the dirty-work to Bilbo. Do not expect this to change. As far as character-development goes, Bilbo is the central focus. He is growing into Gandalf's glowing pronouncement and the dwarves arefor now, at leastbeing themselves.