I think that if importance grinding attack* requires exponential effort then we still can live with it. For example, in Nxt it's possible to increase forging rate by exploiting TF (you don't forge some blocks to be able to forge more blocks in near future). For double forging power you have to pay with infinite amount of electricity burned for prediction. This doesn't lower security of Nxt much. A similar effect may exist in NEM, I need the whitepaper for a deeper analysis.

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* - such an attack is possible, look at my example about Alice, Bob and Charlie upthread and BloodyRookie's reply.