I don’t think there is a right/wrong answer to your question, really. It’s a matter of tradeoffs, and what your priority is.
As @chris mentioned, RAID would help availability on individual node. If there are any issue in the storage part of a node, you would not need to do maintenance from the database side. Thus having RAID helps keep the node from being offline or having to do initial sync which could be an expensive operation that your app can’t afford.
On the other side, not using RAID may help with throughput, as you have mentioned. If this is the must-have feature of your app, then the tradeoff is not having redundancy within the individual nodes and would increase their chances of getting disrupted.
In conclusion, if availability is your main concern, then I would say that you’re not wasting anything by using RAID, with the expense of speed. Conversely, if throughput is your main concern, sacrificing reliability for speed may be a good tradeoff. I don’t think there’s a single correct answer. It depends on what you need from the system.