After this week's blackout's in Spain due to lack of demand response (15GW dropped the Network in like 5 seconds due to a cascading chain of events) it's critical to enable miners to both do Ancillary Services while maximizing the decentralization of hashrate.
PPLNS enables the first part, but not the second one, because if everyone mines separate Block Templates, the pool would pay the same amount to the miner that mines high Fee blocks and the one that mines empty blocks.
That's why SLICE separates both by creating small slices of time that are independent but weighted equally, plus the 8 Block lookback window enables miners to both help the gird (or shut down their rigs for any other reason) and also build their own block templates, independent from the Pool.