John Gruber articulates most elegantly why the combination of external scriptability and internal extensibility make for powerful applications and eliminates some of the demand for open-sourcing those apps. In particular, while I agree with Kevin Burton that an open source aggregator would be nice, especially for research purposes, one with excellently designed scripting and plug-ins would be even better.
On Win32 the external scripting part isn't taken nearly as seriously as on MacOS even though COM is fairly decent for this purpose. Other than Microsofts's own apps, most tools have non-existent to crappy remote object models. As far as I know, no aggregators really support remote scripting. Awasu is one of the few that I know of that has any plug-in model.
If I ever teach my scripting languages class again, I also need to hone in on this point, since such languages are really handy on both the remote scripting (duh!) and plug-in sides of the equation. This generates powerful effects.