At work I do a lot of prototyping against API services that come from both internal and external sources. If you are on a Mac and testing locally a web page that leverages API calls from the client side in the Safari browser, there are no issues there. It is when you want to host these files and share your demo that issues arise. I am not a command line jockey, so configuring test boxes for server-side REST calls is not something I like to spend a ton of time on. So, I often have to rely on a Cross Domain Scripting hack or two for my client-side calls to work.