Occasionally I will do so for a few reasons.
One, and the most important one, is in the vein of “Now, let me see if I got this …” responses.
Similarly, it can offer a different perspective on the implications or repercussions with the proposal of an answer. There, other readers can see it and maybe it will get included in the answer some day. Et c.
I did this for one answer, but it was later deleted. I then decided that I would convert it to a suggested edit so that I could get the author to respond. I rather expected that suggestion to be rejected, as it was rather presumptuous.
Of course, I never did hear back from the author of the answer as to whether my assessment was congruous or not …
I am looking for recommendations on ways I can mark or format such comments in future and not have them misunderstood as — anathema or whatever.