Uploading the texture will block waiting for the GPU to finish rendering with it. Want to overwrite an area of a texture for the next frame while the previous frame was rendering because you have knowledge that the two won't try to read and write the same area at the same time? Nope, tough shit, can't be done. It means that blobs of data can't be placed in the right area of memory because you have no knowledge of whether the memory needs to be for fast reading only, fast read and write only on the GPU, pulling off the GPU onto the CPU etc. It means that uploading data can be delayed until you make a call because you don't know what format it needs to be in. It means that compiling shaders can end up delayed until you actually make a call because you don't know what vertex formats it'll read, what blending modes it'll use, etc.
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