Comment 11 for bug 513460

By selecting a package list, a user could apply an action not allowed for a particular package e.g mark for reinstallation an uninstallable package. That's what broke the "generate download script" functionality.
I pushed a fix in synaptic to check the package flag when a user applies an action to a list of packages.
That should fixes the synaptic issue.

However the behavior of synaptic/apt returning an empty string in DescURI needs some investigation.