I would contend there aren't many instances where you'd want to set XMLHttpRequest to synchronous mode... although Eric is of course correct, if your employer says so, do it
Failing that though, I think what people *do* more often come up against is not wanting more than one AJAX request processing at a time, which is synchronous in a sense too, but not the same way. In this situation you see things like setting and checking flags before sending a new request, locking out the submit button on the UI, things like that. That tends to come up more often.
-- <br />Frank W. Zammetti<br />Founder and Chief Software Architect<br />Omnytex Technologies<br /><a href="http://www.omnytex.com" target="_blank" rel="nofollow">http://www.omnytex.com</a><br />AIM/Yahoo: fzammetti<br />MSN: fzammetti@hotmail.com<br />Author of "Practical Ajax Projects With Java Technology"<br /> (2006, Apress, ISBN 1-59059-695-1)<br />and "JavaScript, DOM Scripting and Ajax Projects"<br /> (2007, Apress, ISBN 1-59059-816-4)<br />Java Web Parts - <a href="http://javawebparts.sourceforge.net" target="_blank" rel="nofollow">http://javawebparts.sourceforge.net</a><br /> Supplying the wheel, so you don't have to reinvent it!