But Bane DOES have a motivation of his own. See? Even in the scenario you describe, bane has a motivation - to obsessively and relentlessly be Talia's protector and ally. She is to Bane what Gotham City is to Bruce.
But the scenario you described isn't a full enough picture. Bane is quite clear about his views that Gotham is corrupt. He calls the stock brokers thieves ("We have nothing to steal!" "Then why are you people here?"). He mocks Jim Gordon and his letter. He views himself as a "necessary evil." He quite clearly has motivations of his own beyond Talia's personal desires of vengeance. One little scene does not change all that.
Right, he has a motivation (the nature of which is leading people to call him a "lackey"), and as has been pointed out...his motivation is a thin one. If that's his ultimate motivation, and it certainly appears to be (other than hating the rich and being really evil), it's only touched on in a single conversation that he's not even a part of.
The problem isn't that the characters have no motivation at all...the problem is that their given motivations are thin, (in the case of Bane's actual motivations) not explored much at all, and in the case of Bane referring to Gotham's "corruption", make no logical sense given what the film actually shows us. The reasons Bane gives for doing what he does to Gotham are nonsense...he's just using that as cover...the ultimate goal is to make Gotham suffer, and to make Bruce suffer through Gotham's plight.