Those who have some non knife-specific martial arts training and who are unlikely to properly drill any of the knife defence advice they glean from Youtube or other sources.
When faced with the immense stress of a knife attack, a person is very unlikely to be able to perform or use un-drilled movements with success if they differ greatly from movements they have already trained to perform.
The video linked in the OP shows a stance that is simple and achievable, but highly unusual (and extremely dangerous in the absence of follow-up movement and strategy). If a person was to respond to a resolute knife attack in this fashion, they are prioritising defence of the vital organs over counter-attack/active defence. The likely outcome is that they will suffer serious (fatal?) injury as a result and - if they survive the initial onslaught - will find themselves in a position in which they are now forced to fight whilst injured and less capable.
A person who has trained to respond to threats with a version of a fairly generic, approximately 45-degree, bladed stance, in combination with a corresponding high hand position (a stance common to many martial arts), may very well cause themselves to hesitate - fatally - if they burden their mind with a new, un-drilled, passive guard which in some ways is the polar opposite of how a boxer, or practitioner of muay thai, karate, (other) krav maga style, or grappling might ordinarily respond to a threat.
If running is not an option, and you are forced into knife defence, you probably have a much better chance of survival/minimising injury if you react in ways which align with your most drilled responses.
Example 1: Boxers do not train against knives, but a basic boxing stance (albeit with open hands) lends itself to very relevant avoidant/counter-attacking movement patterns. This potential strength is almost entirely sabotaged in an instant if when attacked, you suddenly ask your body and mind to do things it has never trained to do.
Example 2: The following example works from the assumption that most knife attackers a person is likely to face will be untrained and likely unconcerned with leg attacks or with attacking a leg artery. Admittedly of course, this may not be the case on some occasions.
A person who has learned to front kick well possesses a useful initial defence against a knife attack if initial range is sufficient (See here). The forward weight of the extended leg allows for a defensive torso lean away from the knife. If you possess this skill already, but sacrifice it in order to employ a technique you've never or very rarely practiced, you may well be inviting a worse outcome.
In short: If you already have some good fighting skills and are interested in exploring the many and varied knife defence strategies available on the internet, consider selecting those techniques which build upon movement patterns with which you are already familiar. In this way, you minimise the risk of 'freezing' or hesitating when attacked and maximise your chances of taking control of the situation, whether this be by overcoming your opponent entirely, or merely sufficiently in order to escape.
To finish up: This is a useful video, less because - if the instructor is telling the truth - it represents an evidence-based approach to an (incomplete) defence against one particular form of knife attack, but because it explains the role of cognition and gross motor skill in combat. Fine motor skills can be trained sufficiently to be of use in combat, but in circumstances where training desire and/or opportunity is minimal, gross motor skills should be prioritised.