Yeah, that Joker isn't very fleshed out... oh wait, we are talking about Bane. I forgot.
In the Nolan movies, most of the villains function as forces of nature / archetypal figures. The Joker and Bane are both Satanic sort of archvillain figures who attempt to push the hero to his limits and destroy him ("speak of the devil..."). They seemingly come out of nowhere, are both animalistic (Joker is a "mad dog," Bane a growling, muzzled beast). Nolan isn't making a Sam Raimi Spider-Man film here (not knocking those, btw). The story here isn't as focused on the villains. It is the effect that they have upon the hero and how he overcomes them that matters in the film. Even Talia is simply a force of nature in a way - she is the cutting betrayal that comes out of nowhere for Bruce, albeit too late in the story to be really effective IMO.
Being a "force of nature" isn't really a valid excuse for being a thinly written character.
If it's the effect the villain has, and not the villain themself that matters, then why even bother to contest whether they're a complex or well written character...since it supposedly doesn't matter?
The Joker, regardless of the idea that he was a force of nature, had fairly clear and relevant motivations that did not rely on another character's. The Joker had a clear reason for doing what he did. The Joker, as a force of nature, also works because he was well conceived and well executed.
See, I don't have to know all this stuff about Bane, just like I didn't have to for the Joker. It isn't necessary in order for Bane to be an effective villain, in my opinion.
It's not about needing to know something. We don't NEED to know anything.
But some level of exploration is neccessary for Bane to be an interesting or a complex character. Otherwise he's just a basic concept.