In Ann Arbor, the "smart" meters are susceptible to an exploit where if you add 5 cents of time to a meter, you remove all the existing time on that meter. For $1, a prankster can reset 20 parking spots and watch everyone get parking tickets. More info at this screenshot of a now-deleted comment on AnnArbor.com.
Sorry, but everyone doesn't carry quarters. I, for instance, am one of those crazy people who sometimes doesn't carry any cash at all, let alone a dozen quarters rattling around in my pocket! Weird, huh? Well, I assure you, I am not alone, and parking machines that take credit cards are a godsend.