Multiple file systems (multiple partitions) have advantages. To me the biggest one is: If some idiot (that would be the sys admin, typically me myself) fills up one file system (like /var or /tmp), then the rest of the system is still working, and you have a much better chance of recovering without drastic measures (like boot from rescue disk, run single user, all those unpleasant and dangerous things). But those are minor considerations. The disadvantage of multiple file systems is that traditionally, they require multiple partitions, which wastes disk space on rounding errors (you never know how big to make them).
Here's a proposal: Why don't you try ZFS on a single partition. That allows you to take that single partition and split it into many pseudo file systems, which gives you flexibility, and the ability to do fine-grained space management. Judging by discussions here, that may be the default setup when using root-on-ZFS in the installer. I don't actually know the details of that default install: While I am a very strong proponent of using ZFS, and have multiple ZFS file systems on my home server, I have not installed root on ZFS (yet).