I have never been a fan of the 'unused' myself. It seems that they do not fall within accepted categories.
It seems easy to make the choice of cancelled or post office fresh (with gum) and for most collectors this is enough.
In some instances, there can be made an exception for some collectors (not all).
1) Rarity, if the stamp collected is rare or hard to acquire, then some collectors will 'make do' with an uncancelled, no gum, version of the stamp. Example: Very old (100 years) stamps that have seen the wear and tear and not survived intact completely. (sounds like I am describing myself here

) or are hard to get at a reasonable price.
The rarity factor also comes into play with some modern stamps too. Two examples are the Canada 2010 Marine Life booklets with their varying cylinder and serial numbers and cover counting marks, and another is the Canada personalized Picture Postage stamps.
On
ebay right now are some Picture Postage stamps that have not been cancelled, are described as used (by clicking the 'used/unused' choices on
ebay's list your item page) and are being bid upon, because they are rare as hen's teeth. I am actually glad that not as many collectors collect these as they are difficult to get now. Now, as for modern stamps, these could be, once acquired, sent through the mails to get them cancelled, if one chooses to do so, so perhaps not the best of examples. Still, I have in my possession non-cancelled and damaged versions of these stamps and am happy to have them.
2) Tropical climates collecting. Stamps in these climates do not keep their gum without things happening to them (tropicalizing, molds, bugs, etc). Some have been issued without gum from the post office (and are noted as such). However, if you don't like a cancelled stamp and want to see a stamp's design clearly, then what are you to do, especially with a limited budget and no dehumidifiers running all the time?
3) And there is the kind of collector who is having fun collecting stamps and has no strong desire to follow any rules or anybody else's ideas of how they should do things (admittedly a rare individual, but cherished) and is going to do it all their way and damn the torpedoes.
4) Also there exists the collector who does not know of 'the rules' or good or bad ways to collect and just likes the stamp(s) and wants to save them. There are probably a lot of these wonderful appreciators out there.
For those of us (including me) who do enjoy a 'proper' used or mint never hinged (unmounted mint) as proof of use or non-use through the postal system then the best that can be done is to fully describe these stamps so that there is no question as to their qualities or lack thereof.
So, one of my Picture Postage stamps that is uncancelled but without gum, would be perhaps described as Canada 2004, undenominated, Scott Catalogue #2063, depicting a cat, no gum, uncancelled.
Some would abbreviate this as MNG (mint no gum) which is a, to me, questionable way of describing it as the word Mint brings to my mind a post office fresh (possibly hinged) stamp.
The question here is proof. How does one prove the stamp is Mint (non-used, not gone through the postal system and oops, not cancelled) whether it has a cancel or not? You have to take the original purchaser's word that is hasn't. Sometimes this is unavailable to obtain, so what becomes of the stamp then?
A lot of the misuse of terms for these stamps and others of the Cancelled To Order (CTO) variety have been made by stamps sellers out to sell a stamp. If a stamp doesn't sell when you call it CTO or Used but does sell when you call it Mint (of some variety) then, gee, OK then, lets call it mint by golly, and see what happens. Just kind of tack on some words to vaguely describe it's non-use so we are mostly covered.
This is where the no-mans-land of stamps comes into play.
Why do we collect used or mint to start with anyway? Aren't some of the terms and all the brouhaha part of stamp seller's mischief stirred up to actually play to our own preferences? A salesman's tactic? Trust? Do you trust a New car or a Used car better? Mind games.
Even with all this deviltry and confusion existing in the collecting world, it is still a great thing to collect stamps and know how they were used and where, and also how they were made and why, and to see fresh examples and also survivors of the years examples.
I Love my stamps!

Note to Admin folks: Can we please have a Heart smiley to use please?