You can tell who runs special-purpose servers, general-use servers, and desktops.
For desktops, there's not really any use to having more than / and swap, although I still use /, /usr, /usr/local, /home, /var, and swap. I like to keep the core OS (/) separate from the OS (/usr) separate from the ports (/usr/local) separate from user data (/home).
For servers, you gain a lot of flexibility by partitioning things based on where disk usage is expected. Sure, you have to do some up-front planning, and may have to re-do things down the line, but the benefits greatly outweigh the risks. And now, with ZFS, things get even better, as you can have individual filesystems for just about everything, tailored to their specific uses.
For example, you can make /usr/src separate, and enable compression since it's mostly text files. And make /usr/obj separate without compression. Same with /usr/ports and /usr/ports/packages. And then give each user their own filesystem, with different quotas and reservations. And so on. Very powerful, flexible, useful stuff.
So, it all comes down to "how will your disk space be used".