Well, it depends really. If people, for whatever reason, want to use a single RenderTexture to draw to multiple texture destinations every frame because they only need a single "drawer" RenderTexture, then it makes sense to enable them to do so. Some time not so long ago, I even recommended reusing RenderTextures because it minimizes the number of contexts floating around, and in the end, it really doesn't make sense to store multiple of them if you can reuse the same one. OpenGL FBOs were designed to be easy to swap out attachments for for this purpose, so I don't see why we shouldn't pass that power on to SFML's users as well.