This way we don't have to include an icon for the installer progress app. There is a minor issue where the dock icon changes back before termination. However, this app should show up in rare cases in the first place, so I still think this is a good trade off.
For most applications, it makes more sense to use the app's icon than Sparkle's. That and we can save some disk space usage.
This created more complexity than it was worth and leads to subpar user experience because the user can see the icon switch back to the generic one when the updater terminates. So we now bundle the icon in the install progress tool. If a developer wants to use a custom icon, they can provide their own customized tool.
Zorg