As someone else put it: The larger volume is really for convenience.
I used to (back in the old Win95 days) create the separate partitions for OS, applications, user data, and even the swap file. It never really bought me anything and it was a hassle to manage.
It’s far easier to just have a large partition and have a directory structure that you can dump things into. You don’t have to screw with bumping into what you thought was a reasonable size limit but turned out to be too small, or the shuffling of files around when you need to grow (which is one of the benefits of Drobo and one of the primary reasons I bought it).
For the most part, if you keep things organized into a single directory (like My Documents in Windows or your home directory in Mac / Linux or maybe even having a seaparte ‘media’ directory for photos, video, and music) you can backup your user data just by backing up that one directory tree (or two if you split out the media). If you need to reinstall everything wipe it all out, reinstall the OS, apps, and then restore your user data and you’re back in business. You pretty much have to reinstall the apps, anyway, due to (Win) registry settings.
For me, I don’t see the benefit, any longer, of splitting it all up into separate partitions. Maybe if you had ‘work’ and ‘personal’ stuff but even then I’d just create one large volume and two top-level directories.
Oh, now that I think of it I guess I could see creating a FAT32 partition for compatibility so you could more easily share files across a multi-platform network.