The biggest problem with this approach is that it doesn’t help much when volumes are low, e.g. in very small departments or overnight centres.
In such circumstances, you are advised to consider the each of the Erlang uplift systems above and see if any fit. In situations where the volume is particularly low, you’ll probably simply have to staff to cover the hours irrespective of volume.
Another drawback is the assumption about taking a view of the “average hour”. In centres with more peaks and troughs a more detailed approach will be required. For most centres, though, this notion of an average hour works extremely well.