As for flash memory:
Yes, I use it as much as possible. I have a Surface Pro Classic, 128GB, with a 128GB Micro-SD card. I try to put as much as I can on the SD card, but:
This is my laptop replacement, so I have full installs of Visual Studio, Office, and some Adobe CC suite apps installed. And that disk is full. I try to keep all my user data on the SD card, but the SSD is still full.
For starters, some things just won't go onto the SD card. Dropbox, for instance, just doesn't want to keep its files on the SD card, or apparently, any other mountable drive. so there's a few GB of my mirrored files that just seem to have to be on my boot drive. In fact, my whole login profile (Users\Me directory tree) seems best suited for the boot drive because if I moved my login tree to the SD card, I'd never be able to log in and swap the SD card for a newer bigger model. So, now I have every app and its brother littering my login directory.
And speaking of apps: download App Store apps, and they go onto the boot drive, as does all of their app-specific data. Like if I want the Metro Kindle app, all the kindle books I download go onto the boot drive, not the SD card. And I got a lot of Kindle books.
I've told iTunes (yeah, I know) to move its library to an external drive, but even still, it finds a whole crapload of stuff that it insists on caching in my login directory tree. And for some reason, SyncToy just NEEDS a few GB of app local storage.
Aside from that, regular windows apps like to install to C:\Program Files. Yes, I could probably install them elsewhere, but past experience with desktop windows has taught me that trying to install applications to anywhere but the default C:\Program Files just puts you in line for a load of troubles.
Then, there's windows itself. For some reason, every time I install Microsoft's monthly load of emergency patches, the disk space used by the Windows directory balloons even more. This is typical of any standard XP/7/8/8.1 windows install, but it seems like the longer you have Windows installed, the more of your C drive it eats up.
Right now, my C:\Windows directory is absorbing 31.1 GB of my SSD, and that's AFTER I moved a whole bunch of garbage in the Windows\Installer\$PatchCache$ off to an archive drive. (About 5GB worth, so if I didn't prune, there's 36GB for Windows alone).
Not even considering apps, I just can't imagine trying to run this thing out of 64GB. Considering apps, between Program Files and Program Files x86, I've got another 32GB, plus another 12GB in Program Data. Yeah, I've blown clean past that 64GB SSD without even logging in and adding any files of my own.
Sure, you can control the Program Files load by not installing many apps, and selecting the most lightweight options of those apps you do install, but you're still staring at half your drive being absorbed by the Windows directory alone.
I just really can't see realistically running windows off a 64GB drive. If you have smaller needs, maybe RT is the right choice. It lets you actually use more of your 64GB. If you're looking for a laptop replacement, or anything that actually, comfortably runs full Windows, I'd really recommend a bare minimum of 128GB, and more realistically, 256GB.