We need different harvesters since if we had only one, that lonely harvester could include only those transactions that (s)he likes. Also it would mean that the chain can be easily overwhelmed since the harvester only has little importance compared to the rest. So yes we need as many harvesters a possible to ensure a strong network.

On the other hand if you have a node that doesn't harvest it still can contribute to the network by helping to distribute the transactions and blocks and that is valuable too.