For individual systems, as opposed to a large network of users, a little program and data organization logic goes a long way...
My 8.1 boot partition (labeled Boot) is 150GBs and I've got 50GBs free...
(I have two physical drives with a total capacity of two TBs at home--no SSD.) Ideally, the only thing that should go on C:\ should be your Windows installation and all of your OS utilities, gpu drivers, core logic chipset drivers, etc. There is absolutely no need to locate user profiles to any other partition than default (c:\) if you are in the habit of good installation policy, which includes creating as many partitions as you require to logically organize your applications & data. I think that SSD's of ~150GBs are fairly common these days--that's absolutely all the space you need, user profiles included. If you have a larger SSD, well, you can simply have an even larger c:\ Boot drive partition, of course!
By way of example, you might have a boot partition (c:\) with Windows and associated programs (SSD), a d:\ partition containing word processors and other work-related programs that you might label "Work," then another partition e:\ for your CAD programs and data files, etc. that you might label "CAD", and then another partition f:\ for your games and related files that you might label "Games," and so on. The point is to choose the number and labels for your partitions that reflect a logical separation and organization for your programs that is based on your personal needs and/or requirements. You'd also proportionally size them each as a percentage of your total drive space according to your preferences and expected capacities. From that point on, no matter how much local drive space you choose to add, programs like Acronis TrueImage will allow you to preserve the number and labels of your partitions in perpetuity, even though you might add terabytes more drive space in the future.
I've been doing it like this for > 20 years or longer (ever since my first hard drive)...
Here are the significant advantages as I see them, as compared to the normal 1 or at most 2-partition layouts. (Having a gigantic c:\ drive for everything is just an accident waiting to happen...
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*Because your programs and data are compartmentalized and organized into separate drive partitions, if one partition experiences a drive error and fails with a read/write error, only that particular partition goes down--the rest of your programs and data are
completely unaffected. If you have one or even just two partitions, then the same event will cost you most or possibly all of your currently installed programs and data. (As I said, a monster c:\ drive where everything is dumped is just bad news waiting to happen.)
*Using separate partitions makes backups an order of magnitude simpler, and possibly much quicker, too, because you back up each partition separately. Organizing your programs and data intelligently into separate partitions allows you far more control in selectively backing up your data, and should you choose to, say, backup only your Boot partition on a regular basis--well, backing up ~100GBs (out of my total 150GB partition size) is far faster than backing up 1-2 TBs to get the same coverage, isn't it?
I think that relocating the user profiles to another partition simply reflects a poor or non-existent drive partitioning strategy regardless of what Microsoft OS installation programs require...
Partitioning intelligently is easier than ever to do these days as Windows (8.1 included) provides standard utilities to make the whole process point & click (See "Computer Management/Storage/Disk Management".) Make your drives work for you instead of against you! If you ever begin this policy, you'll never abandon it.