KWin has lots of cool features, and I've found uses for most of them. However every time I try the manual window grouping in the taskbar (set mode to manual in taskbar settings, then hold alt and drag items over one another) or window tabbing (middle click drag the window title onto another window) I never continue to find a use for them.

Does anybody here use them on a daily basis?

I don't. They sound fancy and cool features but I have found them to have very niche usability and uses.

Like example I use taskbar with full sized tasks as I want to easily click tasks and see what I have there instead just icons (aka Windows 7/8 or OS X).

Having KWin tabs is like question "why I want to group different windows to one?". Example having a Dolphin and Rekonq windows grouped.... it does not make sense at all.

For taskpanel grouping... No use as virtual desktops takes care of that, applications what have lots of content (like browsers) you have tabs itself and KWin other effects like "Present" removed the need to search.

I focus to minimalistic and very logical and effective graphical user interface and groupping and tabbing are "terrible".
Having a window decoration filled with tabs is not good either.