This change butts up against what I think is a fundamental, unsolved UI issue. Power users often don't realize how much if an issue "getting lost" is for novice users. Dave Worthington's Mom said[1] of the iPad:

Well, it’s too touchy. Even though I’m better with it now… if you happen
just to move your hand or something, you know, then all of a sudden you’re
out of what you’re in. That’s bad I think.

Notice she doesn't say "you go back a page" or "you've opened something" she says "you're out of what you're in". Anyone whose done some usability testing knows that users all the time get "out of what they're in" and are completely lost. And as the designer you are screaming in your head "You've just gone to your Account page! Just click the huge red "Back to my movies" link at the top of the page you ninny!" All. The. Time. Bless us, designers and users both.

I call this an unsolved problem because the options are equally bad:

1) Have one panel where you replace the contents frequently. Like a web browser. Unfortunately, as the article points out, prevents doing two things at once. And it forces people to learn often complicated mechanisms for navigating from state to state.

2) Show multiple panels at the same time. Takes up a lot of space, especially when there are lots of panels. Can be confusing at first.

3) Show multiple panels, but with the ability to close panels. This introduces the UI problem of reopening them. You can allow people to shrink/move around the panels independently, but that gives the user even more ways to lose something.

There are more advanced ways to deal with the problem. Zooming User Interfaces[2] were meant to deal with this somewhat... by giving everything a place and a context, they were supposed to allow you to maintain a good sense of where things are. But in practice, they turned out to be even more disorienting than a contextless browser window because navigation is less constrained and there are so many more places to get lost.

Microsoft, with WP7 does a little innovating here with their panoramic view control[3]. It puts UI elements in a context and lets you see a little bit of that context, without forcing people to do free navigation in a virtual space.

As for Skype, I won't argue that the new version is better for anyone, because I haven't seen the user testing. It's obviously worse for one person, but even objectively better designs are sometimes worse for power users who have deeply entrenched workflows in the old product. I do think there's a reasonable chance that they made the change because of user testing and that novice users are fundamentally happier in the new version. Again, I don't know because I haven't seen the testing.

But I will say that the OP is wrong about something. His problem is not the overall decision to integrate the video and chat into one window. His problem is that they hid the "open a chat for this person" button. If they had left the windowing exactly the same, but removed the "open chat" button from the user list and made you mouse over a video window to see it, he would've been equally lost.

The problem isn't that they made the wrong choice among flawed windowing models. The problem is they didn't do the work of making the UI work well within the constraints the windowing model provides. If they did extensive testing with a variety of users (including both novices who get lost, and advanced users who construct massive structures of windows to coordinate with dozens of people) they would've caught the video/chat bug.

[1] http://technologizer.com/2011/03/28/my-mom-reviews-the-ipad-...
[2] http://en.wikipedia.org/wiki/Zooming_user_interface
[3]http://www.kotancode.com/2010/08/12/preview-of-the-panorama-...