Actually I disagree. No one denies that the CANZAC units were probably the best quality troops on either side during the war. But this was for one simple reason, they were volunteers and the had higher standards than anyone else. By this point France and Germany was throwing anyone who could walk, talk and stand up into the line and Britain wasn't far away. The CANZAC's never relaxed their standards to the same extent, leaving their units qualitatively superior. But it also meant there was a finite limit on manpower, exacerbated by the small population of the countries involved.
Now the British and French General staff (who decide if a new army gets formed) knew this. So if you have a diminishing, rare, resource (due to recruitment not keeping up with losses) you need to use it in the most efficient way possible. In this case the same way the British used their best units like the Guards Division or how they used units like 6th Airborne in WW2. As shock troops only put into the line when needed.
Now I know this isn't what happened and CANZAC units were put on quiet sectors and that was a waste of their talents.
So if we stop regarding these units as ordinary infantry divisions but shock division we have two option. To distribute them in penny packets, i.e. one or two divisions per Army or exactly as OTL, keep them in separate Corps and move them around as needed, striking a good balance between concentration and the need to be everywhere at once. But by creating a whole Shock Army you would be throwing that away. Either it is, like every other Army, assigned a section of line, a complete waste or its is held back in reserve, weakening the front line. Due to the nature of battles on a broad front when faced with something like the Michael offensive it wouldn't fight as a Army but instead would have its troops detached as OTL to the command of the Army assigned that section of the front. So such a Shock Army wouldn't be a real Army, instead a holding body for the B.E.F. reserves, a needless duplication of resources as the 5th or Reserve Army was perfectly capable of controlling the reserves without the need for an additional Headquarters.
Edit: I forgot to mention that the Fifth Army ended up under the control of
http://en.wikipedia.org/wiki/William_Birdwood an honorary Aussie and former commander of the ANZAC's.