Is it not remarkable that the Emperor waits until October 1813 to decide that the 'fire and bayonets of the third rank have no effect'? Why so late for such a significant observation?
A battalion in 'column of divisions' - the traditional battle winning formation for the French infantry - would lose mass, reducing from 9 ranks deep to only six. That doesn't sound useful.
Like others in this thread I suspect this has more to do with making shrinking battalions look bigger and take up more frontage on the battlefield.