I don't do it with linode but I like to have multiple partitions on physical computers I admin:

/ - duh
/boot - kernel, grub
/home - home directories
/srv - databases, torrents (only the FOSS kind, I swear!), web content, etc
/usr/local - I don't use it much, some custom shell scripts
/opt/texlive - where I install the TeX Live system

The advantage to having them be different partitions for me primarily is OS upgrade. I can format / for a clean install but leave the others alone.

Since I use LVM (except for /boot - it's ext2 primary, don't even see point of journaling for that) I can adjust partition sizes as needed.

With Linode, I think the easiest way to upgrade is to buy another linode, set it up, migrate data (rsync over ssh, use IPv6 and within same data center it is free), test it, change DNS (or bring up old IPs on new box), delete old Linode.

So I don't see a personally need to use different disk images in Linode.

Backing up data using Linode's backup service could be considered a valid reason to some. I'm sure there are other valid reasons some have as well.

The first time you'll have a runaway process spamming into /var/log and filling your disk to 100% you'll learn to put that directory on a separate disk/partition...
... or not, I stil don't do it after experiencing such incident.

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rsk, providing useless advice on the Internet since 2005.