Former office supply store shill here. Lots of people love the space pen’s outer design since it is so small and shiny, and many could care less about the internals… For them, by all means, throw a knock-off cartridge in there…
But a few artists I know who draw in ball-point swear by the fisher refill for it’s consistent line and lack of “gunkyness.” For them, a gell or roller-ball refill wont do, since those produce a solid dark line straight out of the pen, almost like a fountain pen. Ballpoint ink is its own strange and viscous substance that lets you slowly and lightly build up to the full-strength black line. Sometimes just the bic or papermate disposable it just what you want for drawing.
Ah different tools for different ends 
