i agree that the metal wedge is unnecessary. i have never needed to use the metal wedges in any of my ax jobs, but have used them on same hammers though. in my personal opinion, there is absolutely no reason or need to "lock" the wooden wedge in with a metal wedge if the wooden wedge was driven properly.
more importantly, i feel very strongly that one should not split the grain of the hickory handle. the whole idea is to keep as much wood and grain intact in a striking tool handle as possible.
that last point is why i also advise people NOT to double wedge with wooden wedges also, such as 300 explained, where you cut two perpendicular kerfs, pound in the main wedge, then pound in smaller sliver wedges into the "cross" kerf. with a tapered eye of an ax, there is no reason to need to create a wedge action in the long axis of the eye... and like the metal wedges, you are splitting the "intact" grain that should be running the entire length of the handle(hopefully)
the round metal wedges you see on striking tools today are used in machine pressed hammers and axes, where the head is hydraulically seated and wedged. the round wedge is insurance for the fact that the handles are not being fit by someone spending time to insure a proper fit. they are also used on hammer type striking tools, simply because the eye is so small. the round wedge gives a good wedge action in such a small amount of wood/surface area.
moral of the story, IMO, dont fix what isnt broken. a properly driven softwood OR hardwood wedge will be completely sufficient to out last the rest of the handle in use.