Mapper hacks can
be useful in theory. If you provide documentation (ideally including source code for the modifications and some template ASM that can take advantage of the mapper change), someone can take it and immediately do something interesting. Much moreso if the author does more than the bare minimum. Although it won't be distributed as a patch, the upcoming version of Editroid converts a Metroid ROM to MMC3. In the process it fully expands it, converts it to CHR ROM, adds CHR animation (via cycling) for backgrounds, modifies some code to allow level data to be stored in any bank, and has routines that can be used to safely perform bank swaps (they aren't atomic/thread-safe, on MMC3) so new code and data can be easily accessed.
Now, hypothetically, if I distributed this conversion for Metroid as patch, should it be allowed on RHDN? If you simply apply the patch, you won't notice any difference in gameplay, but if you intend to hack it, it instantly opens many doors
In general, we try not to judge the quality or usefulness of a hack. There are certain exceptions to this (e.g. no half-baked naked mario hacks), but for the most part, people's perspectives and opinions can vary widely so we try to be inclusive. Although you or I might find a particular submission to be useless, somebody else is bound to disagree.
I think a mapper conversion should at the very least state in the description that it makes no changes to gameplay or content, and should document its "API" (i.e. where and how do you call the routines to take advantage of mapper features). I don't see a need to ban them though.