I was a bit confused at first about what sourcePaths did (I though it was the same as a source folder in Eclipse, in a sense), but now I get it. However, I can't say I like the design of it.
When someone defines a sourcePath in a DUB package, in the vast majority of times, they will want that to be in the import path as well (so it has to be defined in importPaths as well). You see, if you pass a D source file to DMD, but that source file is not part of the import path (or it is, but the (full) module name doesn't match the directory structure up to the import root), then that source file is not importable anywhere in D code.
This has to be a very, very niche use case then (I'd like to see examples otherwise). And partially covered by "sourceFiles" already. I would estimate that 99% of sourcePaths will be importPaths as well. And yet for that use case (a sourcePath folder is also an importPath folder) there is no way to do that with just one attribute in the .json. It would be better if we had such a mechanism.
I guess not that many people have been annoyed by this yet, only due to the implicitly defined source/import folders (that is, if you are using the "source" or "src" folders). In this way both sourcePath and importPaths will be the same automatically, because they match to the same folders, and nothing was explicitly defined.