I was offered 2 prematch contracts.
Upfront, both programs asked me if I participated in the match yet. They told me if I had, they couldn't offer me the contract. Supposedly if you're already involved in the match, you can't do a prematch contract because you're bound to go into the match.
I can't say if it happens in this field more than others because its not like I have a statistical analysis that grades which residency field gets more.
When I did get offered the contracts, my feeling on the entire process went from slightly scared to pretty good. From there I knew I was going to get into some place that was at least decent. As an American born IMG, I was fearing getting into a program.