You would not gain much anyway. In fact it would make things slower.
Perhaps you are right, but while we find some measuring data, we just have prejudiced opinions. There is people thinking that more time than suspected is lost in moving heads back and forth when defragmenting big files. I've had this sensation too, and also that dumping from one drive to another, and then back to the first one, is a much more painless operation, but it must be proved.
Someone should build the test, creating a big spacehog (a 800Mb video file for example), divide it in some hundreds of random fragments, defragment it in both ways, then compare times.