Let me explain:
While a character is traversing normal hills, slopes and angled surfaces, as long as their body is oriented in the vertical direction, the camera should also be oriented in a vertical direction. However, as soon as a character begins "wall-walking" (notice I'm not restricting it to mere crawling) then the camera should rotate to reflect a new up.
The greatest benefit for doing this is the ease of controlling the character's movements. From the player's perspective, the entire control scheme is identical to moving on the ground, which simplifies things greatly.
Just because the camera views up as sideways or upside down does not affect gravity at all. Thrown objects, ground targeted abilities, falling and jumping are still affected by gravity in the same way, regardless of the orientation of the camera. This could lead to weird and perhaps even disorienting experiences as an upside down character standing on the cieling throws things that go "upwards". But in my opinion that would be weird and disorienting in a really cool way that I think would make people enjoy it even more.
Furthermore, i think this can be used by the devs to create some interesting carnival house or MC Escher style maps in which players on a team could all be standing at different angles to each other. It could also open up possibilities for vertical and inverted surfaces as viable target locations for ground-targeted abilities, summoning circles and obstacles to be cast on walls or cielings and stay there, oriented according to that surface rather than to gravity.
Followup question: Would we want to do this with flying as well, fixing the camera's up with relation to the character's body? If so, should we implement roll control like a flight simulator?
I like to take your ideas and supersize them. This isn't criticism, it is flattery. I come with nothing but good will and a spirit of team-building. If you take what I write any other way, that is probably just because I wasn't very clear.