Honest question: what's the added value of separate windows (to be precise, separating contact list from discussions) to you?
Same goes for various chat apps that don't open conversations or buddy lists in new windows. Even vim, as much as I like it, frustrates me I can't open views to a buffer in separate X windows, no I must split windows in vim.
Window management is a bolt on feature to these apps and none of them do it as well as the window manager I'm already running.
I wish Windows and OSX had implemented OS wide tabbed windows a long time ago, then maybe these application developers wouldn't have felt it necessary to add these features inside the apps.
"All-in-one" UIs allow for less customizability/rearranging, but IMHO that's a trade-off they do for one huge benefit: simplicity.
With this in mind, how do developers decide which direction to push? By looking at their target user base, of course. And the target user base of Skype is not mostly savvy professionals who like to rearrange windows/widgets, it's humans wanting to talk to other humans. So Skype developers naturally push towards simplifying.
Other examples of applications who are kinda, in various ways, reducing such customizability:
- Office, with the Ribbon: no more detachable toolbars.
- All iOS / Android apps: single-screen, one thing focused at a time (with the exception of left/right tiling in recent iOSes)
- Even the GIMP introduced a Single Window mode in recent versions.
Now, why don't Skype developers let users pick whether to choose single or multi-window mode? (which they used to do in the Qt-based version under Windows, see the "View" menu).
- Because configurability means more code, more tests, more bugs, more money. So if only a tiny part of the userbase requires the configurability, product managers bring the hatchet.
- And again, options means beginners will accidentally change them to the one that doesn't fit them, won't understand what the hell happened and how to change it back, ending up either grudgingly accommodating to the undesired option, or getting frustrated and leaving.
Idiocracy, in action. :-)