Non-quantifyable arguments such as "the game is good enough to sell on it's own" are just absolute lunacy.
I'm not sure that arguments based upon quantifiable, but completely made up, numbers are any better.
If the game will sell itself by selling a single copy, imagine how it will sell itself by flipping that 1 copy into 20, and all the exposure that it gains as well.
And, if the owner of the store purchases a copy for the purposes of generating sales, thereby increasing the revenue of the owner's store, it's potentially a net benefit to him as well. If having a copy of the game available at the game store increases the sales, it's in the interest of each party to purchase it - there's no reason one party needs to give it away for free. If the store owner can sell 20 copies by buying 1 and doing a demo, it's most likely in his interest to do so. I suppose there are always those people on the bubble who would be persuaded by a free game to run a demo, but I don't know that there are enough of them or that they are sufficiently motivated by a single free copy to become an Unstoppable Marketing Juggernaut.
I'd be particularly wary of a guy going booth to booth, asking for free games and promising to demo them at his store (particularly if he already has a stack of games from others who have fallen for the line - unless he's got an infinite amount of time, he can't demo all those games, and I have no assurances he would demo mine in preference to others).