Thinking about it, a "hand-held" blade (as opposed to a wrist- or rifle-mounted one) seems to be more useful - more leverage and flexibility?
Where a wrist- or gun-mounted one would score would be in that they leave both hands free for doing something else: in the case of the bayonet - clutching the rifle, in the case of the wrist-mounted sword when the owner is holding a pistol in the other hand.... er.... something? Enjoying throttling passing vermin? Reading pron? Picking your nose?
I do share the view that given the choice I would prefer using a hand held weapon over a wrist mounted one were I ever unfortunate enough to be in close combat, but it doesn't leave me lying sleepless at night, I must admit. When I first saw it, I thought "ooh, different".
Now on the other hand (to coin a phrase), any kind of mech or walker, that just makes me cringe at the sheer level of impracticality for a fighting vehicle!