You save on the flipside when you need to spend your postmix coins. Say you wanted to send 1BTC of postmix coins. If you only ever used the smallest 0.001 BTC pool, you would need to spend 1000 of those UTXOs to send 1BTC. That would be a very big transaction (as in data size) which would cost a high fee to mine.

IMO its good to have a mix or whirlpool outputs from each pool size.