I think it leaves things in there until something new needs to go in. I guess it's for speed. There's no point clearing out the VRAM unless something new needs to go in there.
I had a thought about streameing in textures ;o) AGAIN
Basically, you break your textures into say, 32x32 or smaller squares and load each of these chunks in, bit by bit. Then, copyrect these to the main texture. Since blitz is loading and putting these to a texture for you, the copyrect command isn't actually THAT slow. Plus, you could use the Force to VRAM flag, beause each texture piece would get freed immediately after copying.
I had a thought about streameing in textures ;o) AGAIN
Basically, you break your textures into say, 32x32 or smaller squares and load each of these chunks in, bit by bit. Then, copyrect these to the main texture. Since blitz is loading and putting these to a texture for you, the copyrect command isn't actually THAT slow. Plus, you could use the Force to VRAM flag, beause each texture piece would get freed immediately after copying.