The purpose of netting is to prevent birds and vermin damaging a roof by pulling out straw from the thatch. Bird and vermin damage is difficult to predict and may affect one thatched roof but not its next door neighbour.
Bird damage is not often a serious problem and often looks worse than it actually. Only the ridge detail on water reed roofs need to be wire netted, As this is constructed in straw.
The whole of a wheat reed roof needs to be fitted with wire netting though. Wire netting should be fitted in a way that can be easily be removed by firefighters should a thatch fire occur.