I've never understood this issue. IMO as long as there are at least 2 adjacent center caps still on, the state of the cube is unambiguous.
No, it isn't. When two center caps of a different colour fall off, it's still physically possible to put them back in the wrong place. Of course, if you had the chance to repair it before you stopped the timer, you probably would do it right, but that's not the point. The point is that there is a possibility to place the center camps on the wrong face and that's a DNF. The regulations assume the worst state obtainable by repairing the repair to determine if it's solved, +2 or DNF. People seem to think it means worst solvable state or what you would have done if you had the chance to repair it before stopping the timer or even whether the rest of the puzzle is solved. The regulation itself is quite clear. It's when people starting to read things into it, that aren't there, when it goes wrong. Just some examples on how I would judge various situations with comments. Not that they're in any way authoritative, but they might be helpful to understand 5b5.
3x3 one center cap off: Solved. The center cap can only be put back on one center, which is the correct one.
3x3 two center caps off: DNF. The center caps can either be put back in the correct face or each other's face. The latter is the worst obtainable state and therefore determines the result. DNF.
3x3 edge off: DNF. Edges can be placed in 2 ways. Correctly or flipped wrongly.
3x3 corner off: DNF. Corners can be placed in 3 ways. 1 correctly and 2 flipped wrongly.
4x4 and higher cubes 2+ center caps of the same colour off: Solved. Whichever way you put them back, it will result in a solved cube.
Pyraminx tip off: +2. Tip can only be placed back in the correct place, but can be aligned incorrectly.
Square-1 any piece(s) off: DNF. Although pieces can only be placed in one orientation, corners and edges are interchangeable with each other.