ZFS on root enables you to have boot environments, which is a functionality originally found on Solaris. Not that they're in scope for this initial implementation in Ubuntu, but they could be built on top. Furthermore, ZFS protects you from bitrot via multiple copies or RAIDZ or at least lets you know bitrot occurred (scrub). It lets you resize the dataset (I tend to undersize partitions such as /boot). I could probably think of other reasons as well, but this would be enough for me.