Hmm.. Ok. I noticed this "do not use your whole HDD"
post, which also says how some larger drives can vary in actual size.
So, rather than say "reserve X" via checkbox or slider or whatever method, I think the real idea is more like "define X as whole disk size" where default is 100% of actual size, and can be reduced to arbitrary amounts, leaving unused space. In the case of ZFS as mentioned in the article above, it allows for precise and equally sized HDDs, apart from what the manufacturer(s) provide(s).
Also, if it is possible to use *whole* disk, then the above as a variation of this should also be possible, but the key being that the unused space is purely unused and always unused (ie. not recovered at a later date for NTFS or some other purpose). In the context of ZFS with its constraints on arrays (I am very new to ZFS so I mostly assume at this point) it makes sense.