Leach got through some overs on the last couple of tours, so having a spinner in the side would seem to be a good idea (notwithstanding that it's always a good idea).And England take three spinners to New Zealand. All ready for the green tops.
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If you want a spinner in the team (and you do) then you need two in the squad in case one of them is rendered unselectable for any reason.
So really, we're just questioning why Rehan Ahmed is there. I guess it's just as useful to have him there as it would have been to send him on the Lions tour in South Africa, and the alternative to either of those options would presumably be that he spends the winter pushing trolleys around Narborough Road Tesco.