What are the use cases of in-do-end?

[GrayFace <sergroj@...>]

So far I've read mentions of several use cases, but I don't understand 
how exactly it may be useful in them. Here's what I read about:
1) Obvious usecase: something like Deplhi's 'with'. This is not the main 
use case Lua developers targeted, for sure. Inability to call global 
functions, a need for additional local variable to call methods makes 
this incomplete.
2) Modules. Maybe it's due to the fact that I didn't work with modules, 
but I don't see it useful. I don't see a reason for setting current 
environment like 'module' does in 5.1 or use of in-end of 5.2 due to the 
same reason: inability to call global functions. Generally, I'd prefer 
to directly write "function P.f()  end" instead of "function f()  end" 
given P is the module table.
3) Sandboxing. I did sandboxing with setfenv and can only think of doing 
it with setfenv or loadin. How can in-end help with it?
4) Some strange use cases of setfenv. What I've read made me feel like 
either setfenv or in-end is used in a wrong way. For example, here 
in-end is used in a wrong way: 
http://article.gmane.org/gmane.comp.lang.lua.general/61606
5) Something else?