I agree with the general request, i.e., offer canned comments to users flagging something as not an answer (NAA), but I disagree with the reasoning.
Why do we need this feature?
When I flag something as NAA, I usually leave an explanatory comment. About half of the times, this is individualised to the answer in question and thus a canned comment would not help me. However, in the remaining cases, the canned comments are spot-on. If I would encounter the respective question in the VLQ queue, I would use a canned comment, but if I am just flagging it (and thus sending it to this queue in most cases), I have to type something on my own. This is unneccessarily tedious.
Due to the above, many flaggers do not comment at all, but only flag the post and hope for someone in the queue to leave an appropriate comment (or do not care at all). This is bad for two reasons:
We want to inform the post’s author about problems as early as possible, so they can react to them and either improve the post to avoid deletion or to delete the post themselves and avoid wasting our time. Moreover, if the flagger does not give a reason for their choice, it is quite likely that nobody in the review queue leaves a constructive comment.
If I encounter a post in the VLQ queue without any comment and know what’s supposed to be wrong with it, I do not have to figure this out myself, which saves time. This holds in particular for comments posted as answers, which at times require one to take the question into account (what is a comment on one question can be an answer to another).
If I disagree with the flag, an existing comment allows me to be more specific in my reasoning against it.
Points to consider
Implementing this feature would make canned comments available to users with 15 reputation, while right now only users with more than 2 k reputation (or 1 k on betas) can use them. Of course any user can post the text of a canned comment at any time, but it is very likely that new users do this. Rather, it is conceivable that inexperienced users misplace canned comments and thus irritate, deterr or misinform other users. However, I do consider this a risk worth taking (and we can still make those canned comments only available to 2 k users, who can use them anyway).
Why do we not need this feature?
(Not: Why is this feature a bad idea?)
Reviewers have to choose between not leaving a comment at all (which may mean that the user doesn't know why their post was deleted), or leaving a canned comment that says essentially the same thing that someone else has already said.
The canned comments in the VLQ review dialogue are really just that: canned comments. There is no difference between manually commenting and using a canned comment. So, if you encounter a post in the VLQ queue that already has a constructive comment explaining what is wrong, it’s absolutely fine to upvote that comment and select no comment in the canned-comments dialogue. The disadvantage of the lack of the canned-comment dialogue for flaggers is rather that there may not be an existing comment in the first place (see above).