There are more to Sister of Battle than the flame throwing/bolter shooting maniacs.
Enough to introduce new weapons or give new playstyle to current weapons, as well as fill the Commodore (and Commissary) while making a full talent tree.
This would still mean having an arm devoted for melee and 1 for ranged, thus still having problems with parts where the hands are required.
The Machine god doesn’t really care about it’s rejects, much less than it does for Skitarii for example.