^From what I see, Law's Power is like an operating table, but doctors need other tools besides just and operating table to operate, so he uses his sword as a scalpel. He can still move things around on the table, as he has his hands, but can't cut flesh with just his hands, as that would be too messy. He could probably use other doctors objects with his room power as well, like chemicals or needles, though I don't know what that would do.
Aggravate, Anger, and Piss off
A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.
thats what i was getting at. (more or less) his power is not linked to his sword. it works well with it but any bladed object would probably be just as good. like a scalpel.
^ exactly there is no real rhyme or reason its just there like a habit or by choice or something like that.
Additionally, Law can control his DF power without the sword, as evident back on SA when he dissembled the marines without drawing his sword. His hand control is part of his operation within the spectrum of his power (see what I did there ). While his sword undoubtedly lends additional speed, precision, and lethalness to his movements, Law remains in need of his hand gestures to manipulate his target. This is similar to Donflamingo's manipulation of his DF via his hands.
I Love You!
Law will just put the strawhats back together after Luffy interrupts. Maybe he only swapped them around because of the situation