I tested your code both with mkii and mkiv, and everything seems to work correctly, both with frame= on or off.
\setuppapersize
[A4][A4] % or [A4,landscape][A4,landscape]
\setuplayout
[header=0pt,footer=1cm,backspace=1cm,topspace=1cm,width=middle,height=middle]
\setupfootertexts
[presentation---\currentdate\space---\space\pagenumber]
\starttext
\combinepages[slides][nx=2,ny=3,frame=on]
% or
% \combinepages[slides.pdf][nx=2,ny=3,frame=on]
\stoptext
that independently of the SimpleSlides module one can combine any PDF file in a handout… Thanks very much!