If I'm buying an S&P 500 ETF and the index is at 6015, then I want my ETF to be trading at $60.15, not $59.45 plus expenses. In other words, I don't want my ETF skimming off the top. Has nothing to do with liquidation or book values.
ETF's get to skip a lot of rules. I'm just not a fan, although admittedly, it is easier to use....and mutual funds are becoming a thing of the past. All the mutual fund companies are switching over to ETF's now. They say for tax reasons