Let's look at it this way ...
You do not want to lose any of your personal files (mp3, jpg, docs, etc etc) - everything else is replaceable at little or no cost (timewise).
The problem with personal files (at least if they are in your $HOME) is that they are severely corrupted by a proliferation of dot-files; so personally, I keep my personal files away from $HOME - ie on a separate partition. Everything else is recoverable. This leads me to the following layout (but bear in mind that I always have more than one distro on each computer - more typically, 3!)
This would lead to the following partition layout:
#1: 100 Mb, ext2 (for legacy grub which I use to boot all others by chainloading)
#2: =memory (swap)
#3: 20 gigs, linux #1 (ext3, ext4, jfs, reiserfs - depends on distro)
#4: extended - remainder of disk
#5: 20 gigs, linux #2 (ext3, ext4, jfs, reiserfs - depends on distro)
#6: 20 gigs, linux #3 (ext3, ext4, jfs, reiserfs - depends on distro)
#7: remainder of disk, (ext3, ext4, jfs, reiserfs - must be common to all distros), mounted as '/work'
/work have the following directories:./scripts, ./bin, ./tmp, ./Documents, ./Downloads, ./Music etc etc and I delete the same directories in $HOME and make them a link to those under /work.
This way - absolutely all my data are separated from the OS and seeing that dot-files/directories seldom are portable across distros (unless, of course, you happen to have the same version of apps and desktops across different distros - in which case I don't see any need for more than one)
Also - each distro is self-contained on one filesystem and it will install its own bootloader on the root filesystem (be it lilo, legacy grub, grub2, whatever). Can you imagine having /var, /home, /boot, /usr/lib etc separate for each distro? Heck, you would soon run out of partitions and it would be a nightmare to support.
So - backups are e-a-s-y to do - you really only need to back up /work. To back up the root filesystem of a distro takes wayyyy to much space and time and unless you have gone to extreme length of making it right just for you. In any case - backing up a 'running' filesystem is bad (imho) - you will find that /proc itself takes about 2 gigs if it has been running for any length of time - and /proc gets reset at each boot - so there's a lot of wasted time and space gone allready.