What are the drawbacks of using synchronous ajax call?

Question

This question could surely be applied to jQuery but in this case I am referring to Prototype. In the Prototype doc it says,

Since synchronous usage is

Answer 1

Well, in addition to the fact that it is very not JavaScript, it has such pleasant effects as COMPLETELY BLOCKING THE BROWSER'S UI. That is not a minor issue.

Did some research, found cool things. Prototype bases "asynchronous" off of XMLHttpRequest.open. According to Wikipedia, this is not something which is not a required part of the specification and it will prevent "onreadystatechange".