I've come across a few people that used them for purchases, but it doesn't seem too popular.
I try to bring a couple hundred dollars in cash (lots of small bills) and have stuck to using the Universal MC for purchases. For a typical day in the park, I'll bring $40 for small purchases, under $10, just to speed up the process of buying drinks and snacks.
I used to use the debit card, but our bank has a maximum transaction fee of $400 a day in purchases. On our honeymoon, we went to purchase tickets for Epcot and the card wouldn't work. Our transactions from the past 3 days all posted on that day. So, our card was denied for direct purchases or cash withdrawls. And we didn't have a credit card at the time to fall back on.
Plus, I'm very anal. Coming back from a trip and entering all the transactions into your check book really sucks.