Is the market response commensurate to the delisting news? In short, is Gridcoin really worth 30% less now? I don’t think so. Clearly the value intrinsic to the blockchain itself is unchanged, since nothing in the blockchain fundamentals changed before vs. after the price drop. So that’s not a factor. It is true that presence on an exchange increases the value of a coin by a small amount, via increased liquidity, visibility, and potential commercial prospects as the exchange expands its operations in the future.

I don't have a background in economics but I have a simple explanation that may be valid.

My thoughts are that the closure of the Gridcoin market on Poloniex created a supply of coins that was not balanced with an equal demand.
There were probably many Poloniex users who had invested in Gridcoin without having a wallet outside of the exchange, these investors would most likely have been willing to sell their GRC for a lower value than the price on other marketplaces in order to convert GRC to another currency before the market closure deadline.
Unless there were buyers who where willing to collectively purchase all of the available GRC from Poloniex the market value of GRC was sure to drop.