Just to be clear, the CCM doesn't "ban" cluster bombs, it only bans a few very specific cluster bombs based on sub-munition size/weight and fusing system. In fact the UK still uses quite a few "unbanned" cluster munitions as the ones we use fall juuuuust outside the scope of the CCM. So for example we still use the M270 MLRS rocket system which drops almost 8,000 sub-munitions per vehicle and no-one complains.
The reason we still justify the use of certain cluster munitions is that their design makes them the most effective weapon for killing large numbers of infantry in the open or destroying large numbers of soft-skin vehicles.