Actually....Windowblinds has too extremely practical functions in particular, which are 'necessities' essential for people who do what is called 'Extreme Customizing', that is, People who replace the Windows shell completely by adopting an alternative.
The locking on top function, and the roll-up.
Even if you were stuck with the same old predictable WindBlows default window graphics [which you aren't, of course] it is a must-have functionality enhancement.
Having the ability to embed other controls, such as WinAMP or a quick-start drop-down menu just prove its versatility, allowing Skinners to experiment with all kinds of tweaks. I think I managed to be the second person to complete a WinAMP-controlling one....it was really clever and, like many other enhancements provided by, and for skinners, gave us more scope for diversity.
I would imagine that the three most well-known skinnable applications on the planet would be, in no special order...
Winamp
ICQ Plus
WindowBlinds
Only one of those would be considered 'indespensible' to people wishing to actually have their Windows Experience skinned...