One argument for leaving the answer as an answer, rather then converting it to a comment, is that conversion takes away an opportunity for newcomers to earn reputation points -- the immediate problem being that it takes 20 reputation to be able to talk in chat (and 50 to comment everywhere -- see most recent update at bottom of this answer).
This means that if a cousin shows up who is a complete SE newbie, we can't invite him/her to a chat room to explore side evidence that might not be directly relevant to the question, or encourage them to stick around and participate more fully in the site.
On the other hand, the answer as posted is not an answer, but instead, asks another question. I haven't searched Meta for this, but I have now seen multiple instances where I want to take an answer that someone has made and turn it into another question, linked back to the question where it was originally posted as an answer, with the original authorship of the question retained.
If we could do this, it seems like a win-win situation to me. It would generate new questions, which could then be addressed directly, instead of having low-quality answers on the original question, or long drawn-out discussions in comments.
Upvotes would not give the new person the same amount of rep points as leaving it as an answer, since questions don't get as much credit as answers, but it would be better than zero, which is what happens now when the answer gets converted to a comment.
Update: I have written a welcome comment on an answer to Brierton/Brereton Spelling Causing issues -- I'm leaving a note here as a reminder that if no edit is made to the answer, it needs to be converted to a comment. As a compromise between the 48-hour window proposed by Ana, and the longer period proposed by PolyGeo, shall we say, give people a week to come back and edit? We can always narrow the time period later.
Yet another update: I didn't realize until recently that new users cannot post comments everywhere when they first arrive because they don't earn that right until they have 50 reputation.
I'm sure Stack Exchange has a good reason for setting things up this way, but it makes things especially awkward for users who are coming to G&FH.SE as their first SE site. Look at it from their point of view: we won't let them comment right away, and if they post their comment as an answer (which is all that they are allowed to do) then we are forced to complain at them that their comment wasn't an answer.
I get it that we want them to jump in and start answering things and asking questions so they can earn some reputation, but it isn't very welcoming to say "you can't talk to unless you jump these hoops and earn 50 points first".
As an example, see this answer which has been flagged as 'not an answer' after I left a comment welcoming the user and PolyGeo edited it to be more answer-like. So my question is: How many 'not an answer' flags should the mods collect before we pull the trigger and transmogrify the cousin-bait, "I'm interested in these specific people, too" answers into comments?
I'd also like more answers from the rest of the community about how long a non-answer should age before it is converted to a comment. Two weeks? One week? 48 hours? Something else?