Simplicity is better than complexity.

You are right, it is an error case to handle; but then creating or renaming a file is already allowed to fail for all sorts of reasons, so all programs already have to check it succeeded and handle errors gracefully. Besides, EINVAL is already returned for bad characters if the filesystem happens to be one that disallows them (like FAT or indeed most other non-Unix-native filesystems), so apps already have to handle that error case too.