Interleaving is used if the drive electronics is too slow to process and store the date picked up by the drive head. This was the typical scenario for old (very old) harddrives. The limiting factor today is the platters and the rotation. Today's controllers are so fast, that much of the time they just wait for data from the drive head. Hence, all modern drives run with an interleave ratio of 1:1.
For more info check <A HREF="http://www.pcguide.com/ref/hdd/geom/tracksInterleaving-c.html" target="_new">PCGuide</A>
<i><b>Engineering is the fine art of making what you want from things you can get</b></i>