[dnanian]

Well, I go into detail in the User's Guide. But there are three main reasons.

First, you can't start up from a disk image, but you can from a partition.

Second, disk images are "removed" one level from the actual disk. So, you have one whole additional layer where problems can occur.

Finally, if a problem does occur that makes the image un-openable, you can't recover any files from the image, rather than losing just the damaged files.