Me and my brother used to get a ton of these as kids. From all over, not just WDW, though they were of the majority. Pins weren't really a thing until the late 90s, and even then, they didn't really make a big deal about them as they didn't encourage trading (pin stores at Disney Village Marketplace and MGM's Sorcerer's hat did not exist).
What made the pennies so special was that, like you said, they cost 51¢. How much does your average pin cost? Last march I was at the parks and noticed how little there was for children to buy. I remember seeing $3 figurines in the 90s. I guess they assume parents are going to buy them everything. But I was one of those lucky kids who's parents handed them a hundred dollar bill, and told me that I could buy whatever I wanted while at the parks. That was part of the fun, and taught us about money managing. But now, park inflation, even during the slowest time of the year, was so great, that if those $3 toys weren't there, I wouldn't have been able to buy much.
Anyway, 51¢ was plenty cheap. But as I noticed when visiting with my own daughter, this is 2015. Who carries cash? I sure didn't while I was there. Simply didn't have room in my fanny pack. Credit cards and ID were stored in a rubber band and placed in it without a wallet. But locally I learned to keep coins and small bills for things like those quarter toy dispensers and mechanical ride on toys. So, I recommend people to remember to bring quarters and shiny shiny pennies for their trip. Also be on the lookout for special $1.25 machines. They press quarters.
