Actually, OBG, you've hit on one of the reasons given for why the US Mint keeps trying to popularize a $1 coin--inflation. Instead of the long gone days of being able to use a $1 coin and getting change back, as you mentioned, the reason is sort of the flip side. Now that items in vending machines routinely cost more than $1 and since the machines accept paper currency, the need for returning $3+ in change to consumers is a headache for both the consumer and the people who maintain the vending machines with enough change. Much easier if the machine can return a single $1 coin versus 4 quarters.

Anyway, not the only reason, but one of the reasons for another $1 coin.