I have experienced the same with a fast installing app and agree that it can probably escape you too quickly. It's also a valid point that you wouldn't necessarily want to add the app to your launcher immediately upon installing it, and may do so later after you try it out. This does seem to invalidate the current functionality in many use-cases.
It appears the other bug # 670403 has a few trains of thought on how to implement.
I believe we should keep the current implementation for convenience (i.e. when you're installing an app that you know you want to add to the launcher) but also add another place in USC where you can choose which installed apps to add to the launcher, at any later time.
However, I believe this begs the question of whether this is functionality that should exist in USC or in Unity (or both?).
After software has actually been downloaded/installed/upgraded via USC it would seem that USC's role is over - I'm leaning towards this actually being something that should be achievable via Unity directly as it seems like quite a significant usability issue if we were to force users to open up USC just to add a new icon to their launcher. It would be a matter of convenience for a user to be able to achieve this in a central place in USC but I think this should be secondary option only.
Just my thoughts.