The advantage of having an unlimited edit time is going back and fixing old posts after software updates. Many times when doing a search I'd run into one of my old posts that had some garbled text or missing images, so it was nice to be able to edit and correct those problems. I see fairly recent posts that haven't survived the merger very well and the ability to edit them, if the author chooses to do so, would be nice. On some other non-auto related forums I frequent they have an unlimited edit time that hasn't seemed to pose any problems.
Just MHO and not a deal breaker on any level, but I found the old 20 minute restriction pretty limiting on the (old D.C.) forum.