I don't know what most miners are doing with mined coins, and I don't suggest dumping them for BTC on an exchange, but if they are being kept or donated to the MARS project (i.e. held), that does not help the coin's liquidity.

Exchanges will de-list any coins that are not actively traded, and for Poloniex that appears to be around 0.1 BTC volume per day. Coin's without an exchange listing don't have a rosy future.

I suggest miners should pass coins through an exchange before making them illiquid, even if it means setting up two accounts and trading with yourself. The exchange commission is very low, 0.2% + 0.01MRS on withdrawal (Poloniex).