Since the comment edit ability was added, the window has always been 5 minutes. The main reason for the pencil icon was to protect Obama. But I really don't think people are so trolly as to abuse comments in this way. It's just such a silly thing to do (change your comment so it looks like some one else has an angle on something that they do not), that I don't think it's ever happened.
So the pencil icon is just, there. I don't think it displays useful information though. The only problem with the pen icon is, you might not want to fix that typo because of the pen.
- Should I edit it, and have the pen?
- Or just let the typo stand, and not have it?
1) The comment editing window is short enough that most edits happen before a response occurs. I've never had to edit a comment after someone responded to it, nor have I really wanted to.
2) If I ever edit a comment, it's to further refine what I was originally saying or fix a typo, not radically alter my stance.
3) It's slightly inconsistent to not record any indication of an edit history for quick edits to questions and answers that occur immediately after posting (which also potentially have the Obama problem, but that doesn't ever seem to appear), but to have a pen indicator for comments.
I just don't think any users try to change their stance very radically in a 5 minute window. If that ever occurs, I'm pretty much the second participant in the conversation will call the first person out on it -- the window is 5 minutes after all.