My main cause for concern was performance - backing up the \appdata\ folder took forever, with severe performance dips for smaller files (I'd estimate a mere handful of files per second for anything with truly small file sizes). Logging in to the original user after restoring it also went through 5-10 seconds with only a black screen and mouse cursor before loading the desktop and UI, which might have been because of the restoration stuff, but was still worrying. I didn't look at SMART data, but even if the drive is just dead slow without being broken, I'd argue that's sufficient reason for stuffing a cheap 240GB SSD in there. Better safe than sorry.
Tiny files always take ages in comparison; unless you're used to what to expect with the USB speed and particular storage device you're using as well as the platform you're copying from, YMMVG. When I back up user profiles, I have a list of about 20 folders that I have robocopy exclude simply because they take freaking ages to copy and their contents are of no value whatsoever (browser cache folders for example); and way longer if copying to a USB flash drive. There's one folder called 'CryptNetURLcache' for example that can take ages to copy in some circumstances.
An SSD upgrade is always nice though
The very fact the profile was damaged before tells me the HDD is going south. I would have that replaced ASAP.
I've employed the solution described in this thread probably 20 times in total (it is pretty rare, whereas I've probably replaced ten times as many faulty HDDs), and only two occasions I recall being called out for this was the problem likely to have been caused by a faulty hard drive. The first few times I encountered it I thoroughly checked the disks for problems because I thought as you do about it, but no problems then or shortly after.
The thing is, when a modern version of Windows (Vista and later) signs out a user profile, it makes entries in the event log if it had any trouble saving the user's registry data on exit (for example, if the security software had locked a particular registry value, or some systray app). Many machines will list this entries without any problems happening at any point soon. I can only theorise why the user profile sometimes get locked out, but I suspect it's because Windows basically said "enough is enough", perhaps because the user profile hadn't been fully signed out and user registry completely saved in any of say the last 10 signouts, and so Windows marks the profile as dirty and moves on. Perhaps there's a similar failsafe system to when Windows 'Automatic Repair' kicks in, because that certainly doesn't kick in every time Windows fails to boot, but obviously there must be some logic to dictate when it does, and it's not necessarily related to a dodgy disk.
Furthermore, if it was a dodgy disk often causing this problem, then it would surely result in the user profile still not loading after the registry entry has been changed; if Windows said "that profile is borked because I'm having I/O errors when trying to read it and I've had enough of this BS for one day!", that's not going to change simply by changing that registry value.