No..we can't create Main Window in smartform as it provides 
the features of LOOPing the data which flows on every page 
created which is not possible in scripts. Tihs is because 
Looping is not possible and is done by Main Window.

it is possible to create smart form with out a main window.
in case conformation of a mail to client or customer there
is no need of main window,this can be send through smart
form with out main window

Yes, that is the Adv of Smartform

YES WE CAN CREATE A SMARTFORM WITHOUT A MAIN WINDOW.....
THE REASON IS IN SMARTFORM YOU CAN DIRECTLY LOOP BY RIGHT-
CLICK ON TABLE...WHICH GIVES US A LOOP WINDOW AND APART 
FROM THAT WE CAN CREATE WINDOW AND A TABLELINE OUTSIDE THE 
MAIN WINDOW...IT TABLELINE WE HAVE A MAIN AREA WHERE WE CAN 
GIVE ALL OUR DATA OVER THERE...
      I SUPPOSE THIS IS ENOUGHTO CONVINCE YOU THAT WE DONT 
REQUIRE ANY MAIN WINDOW IN SMARTFORM...

Purposse of main window in smartform is to automaticlly 
trigger next page if tha data in main window overflows.
  that is why width of main page is same in all windows but 
its height may vary.

   Now coming to the above question if while developing a 
form you are sure that all tha data in smartform is static 
i mean (amount of data is static like fixed no of rows are 
required in all cases ) and there is enough space in form 
to accomotate that data then there is no need to have any 
main window.

But note that a page without a main window can not define a 
anext page . Also command node in the samrtform can only be 
added in main window which will tigger the next page on 
certian condition.