Somehow i'm not following you... I'll list my personal thoughts and reasons.

First if there's a hidden partition that's suppose to be the base install, that doesn't ensure the remainder of your files are safe, plus i have no clue how to access/use those partitions so i end up destroying them for more storage space. Let's not forget if Windows 10 overwrites it, you can't revert using that anyways.

Second, backing up and restoring depending on how much data you have can be an enormous amount of content and more work than it's worth. My main computer (doubling as a file server for our house) has a 1Tb drive that's 99.9% full... If I reset, i'll likely be dumping all my files and installing linux afterwards... (Although i still have to deal with the overheating issue the CPU has)

Third, reinstalling an OS is time consuming. You're looking at usually at least a 4 hour project, tack on one or more failed attempts that could be a few hours per (or longer if it downloads the image as it tries to install); Once the OS is installed you have to install the programs; it's far faster to swap a drive.

Fourth, most computers that you buy brand new that come with windows (not sure about macs) don't come with a OS/Restore disc anymore. Often without the tools to fix it you'd have to call in, ship your whole computer to them, then have them flash the drive with the default installation.

Maybe i'm missing something, but i just see it as a big pain no matter how you look at it.

The small hidden partitions are used only for some end-game restoration workaround. They are usually around 300MBs, you're not saving any space when you're deleting them, as Windows can't expand backwards on a disk, which is usually where they are placed.

If those hidden partitions are several GBs, you can safely remove them. You don't need that junk anymore. Clean and free from OEM bloat ISO files for Windows 7, 8.1, and 10 are available on Microsoft's site.

Upgrading 7 - 8.1 to 10 from the actual disk and not an external device goes super fast. You're looking at 2-3 minutes preparation, and 20 minutes maybe for installation. 4 hours? 10 year old 4800 RPM disk?

I'm adding in the time you need to install software, download&install drivers, changing personal settings, disabling services you don't need; Windows also loves to require about 4 reboots during the install. etc.

A while ago i estimated it usually ended up being 4 hours. Maybe it takes less, but i certainly wouldn't set aside less than 4 hours for it.

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Some things to be aware of if you're concerned about your privacy:

http://www.polygon.com/2015/7/31/9075531/windows-10-privacy-how-to

There are ways to remove the aps. Cortana however more difficult.

http://www.gog.com/forum/general/is_windows_10_mandatory/page5

I've linked some sites that might help you remove MS snoop aps. Some of them you can uninstall like bink bing whatever its called :P

Here is another link a little bit more clearer.

http://www.thewindowsclub.com/uninstall-universal-apps-user-account-windows-10

Here is ways to disable data logging in windows 10

https://www.reddit.com/r/Windows10/comments/3f38ed/guide_how_to_disable_data_logging_in_w10

Here is a way to restore GPedit.msc in windows 10 Home so you can access your Group policy editior

http://www.rushinformation.com/windows-find-gpedit-msc-error-windows-78-fixed/

See guys there are ways to help you turn stuff off. Takes some reading in learning and understanding of what you want windows 10 todo for you.