Would there be any reason why I would want to give /usr /var /tmp their own parition vs. setting up only /boot, swap, and leaving everything in / ?

You could use /tmp on it's own partition and achieve something, if you used 2 linux systems (for example), and mounting that one single tmp partition for them both - that way you would save space for using only one /tmp instead of two. This, I think, is the main reason to make "own" partitions for such systems. Also, if you put /home on it's own partition, you can install a different Linux distribution and format root partition (which might really be needed), but leaving /home as it is, saving your personal data for the new system.

I think it depends on whether you want multi boot or not.

There is nothing wrong in using two partitions

One for / and one for Swap.

You can put your data in a neutral partition, say in FAT32, so that it is accesible to all systems.

Remember one distro may fail to kick start all your hardware and you may need to try another.

I have one partition per distro and a common swap for all of them. Never met a Linux refusing this arrangement.

I have to say that it totally depends on you. The only requirement for every linux is a swap. And all distros can use one swap.

But, there is good justification for having a separate /home. You can do that if you are one who would like to try out several distros.