Wine must contain an implementation of WMF because it is tightly integrated into the public gdi32 API. Some Windows applications are known to play back WMF files one record at a time and depend on the device context where they are being rendered to change exactly as it does on Windows (and this detail is, itself, different between 9x and NT). For compatibility, Wine needs detailed control over its WMF reading and rendering, and it needs to expose this to apps that will misuse it in strange ways.
OleLoadPicture* does not need such things. If oleaut32 were the only place where Wine needed WMF support, it might be able to use an external library. Given that we already need to have an implementation in Wine, it makes sense to use it.
Vincent Povirk