Comment 1 for bug 830492

 (a) synaptic needs an exclusive lock in order to work. If you install a package while running synaptic, synaptic might produce random garbage, crash, or whatever else, because the cache it uses does not reflect the reality.

 (b) Locking single download files would be too complicated, we can either lock the complete download stage or not. Otherwise, things start getting crazy when two applications want to download the same file.

 (c) If you are updating your system, you obviously can't do other changes in the meantime, as then the system state changes, and the update can get broken. Imagine you install B while upgrading your system which in turns installs A, and A conflicts with B. Now, if you install B in the background, APT would try to install A as part of the upgrade and fail.

Not locking downloads while installing might be possible to add at the moment, but then there's also work to download & install in parallel which renders this impossible again (because we need to lock downloads and installs, as we are doing both at the same time).