Isn't it simpler to infer backwards compatibility from proper major.minor version number bumps? A major version number bump indicates API(/ABI) breakage, plain and simple. A minor version bump is simply a backwards-compatible change to the existing major.

Version numbers for marketing purposes are not really that useful, but if they are strictly used to indicate compatibility, they are very helpful. No one cares if the API is at, say, version 1023.301 as long as they know that 1023.200 is compatible with the latest version and 1022.499 isn't.