Why Update Manager doesn't use indicators?

Informing about system updates seems like a perfect match for an indicator (it could appear or change color when there are new updates). Instead of that we have an annoying popup window, which demands immediate attention. I cannot imagine that nobody thought about that before, and it doesn't seem difficult to implement, so I assume there has to be a reason behind that?

There is an indicator in 11.04. It shows gray for suggested updates and red for critical updates. It works in classic Gnome, and in Unity as well. I can't remember if it was in 10.10 or not.

The notification for updates is the sun-like icon in the middle. This is actually displayed because I have a package manager open, but the optional update icon is the same. It shows in red when there are critical updates. I don't see any settings under updates that control whether or not this is displayed, is I'm not sure why sure why you don't see it. I'm set for auto-download, manual install, but that shouldn't matter.

I'm currently developing an application which does just what you asked for, mainly because the update management of ubuntu is not satisfactory for me:

You can download the sources at Launchpad (External downloads). It's in an early development stage and might still have bugs. But it works for me :-)