When should I really defrag

That wording is throwing me. Are you simply describing a case where the pagefile is 1.5GB and fills its own parbreastion? If so, I'd be interested to know the advantage in comparison to setting min size = max size = 1.5GB and putting the pagefile on the system-boot parbreastion. The later approach should prevent fragmentation, buttuming there was none at pagefile creation time or it was defragged just that once.

Using a dedicated pagefile parbreastion is advantageous in scenarios where the user wants the pagefile to be resized without fragmentation. But having that parbreastion on the same drive as the boot parbreastion brings with it the potential for increased seeking and reduced performance.

If you meant closest to the center of the drive (spindle), I'd be interested to know why you think the first parbreastion would be likely to be located there and why you would want it there.