Although Coordinates are listed as a part of the command "/spawnpoint" you cannot use them.

Even if you type the command correctly it will "tell" you that you have set the coordinates to the ones that you chose, but upon death you will spawn at an older/original spawn-point

this happens when you type the command or put it in a command block.
it succeeds (as a command) with these examples:

/spawnpoint @a[x=X,y=Y,z=Z,r=R] X Y Z

/spawnpoint @a X Y Z

and displays the message (set (players) spawn point to the said coordinates)

however when you die the spawn-point isn't updated to those coordinates.

its only when you leave out the coordinates that "/spawpoint" command will work...

which isn't helpful when making certain adventure maps. ( and quiet annoying.)