It's a feature I used on my old diary site, also mainly for backup
purposes. I could select the original template immediately if I botched
the new one.
As for a different template for each entry that would not be too hard
to do with the backup templates, but think about if you are browsing
through a persons blog and it has 3 different templates that are
extremely different, that would be kinda of annoying to me and I would
probably not want to read any more from that blog/blog site.
The backup is definately a good idea though and one we will look into.
Currently you can do that too by changing to a different standard
template, but then all your customizations for the formerly used
template are gone since it simply switches to the basic one. So it
would be like that, but then with a custom template.