Is there some reason I should use
Yes, the image uploader tied to the WMD Editor toolbar uploads to Stack Exchange's imgur account which never expire. In addition, the team routinely backs up the images to Stack Exchange's server, so there are redundant back ups of the image archive.
Imgur may claim that at least one view every 6 months will result in the image not getting deleted, my personal experience has proven otherwise and I've had to re-take & upload at least 10 images by now.
Should anyone edit posts just to reupload images that are already at imgur?
I don't see why not. One of the main pillars of Stack Exchange is that it's an collaboratively edited Q&A site. Anyone can make edits and is actively encouraged. And with Gareth taking extraordinary steps to ensure there is no image/link rot, I really don't see why it should be opposed.