Why?

I have enough bandwidth (100Mbps).
I've got idea from pdumpfs.
I can afford enough disks.

So, I think if all versions of debian mirror are preserved, it would be useful.
- You can find any older version of package when you get broken package and want to back to safe version
- You can use it as delayed unstable.
- It is also useful for jigdo (no package will disappear)