Back in the hey-day of the arcades (at least in NJ), it was expected that if you wanted in on the next game, you put your quarter on the glass.
When the current player finished his current game, he would relinquish the machine (if he had no more credits), or would then allow the other player to join as a two-player game.
Optionally, if you had an extra credit, you could simply take the other guy's quarter and walk away (exchanging your credit for his quarter). This behavior disappeared eventually, when the pricing started changing (i.e. 50 cents per game / 3 for $1) and when the replay scores started becoming progressive.
Now, when I have remaining credits on a machine and have to leave, I will try to find some young kid (who may not have even normally tried pinball) and point him to the "free" games on the machine.