Enh, maybe just for the sake of completeness. I also believe there are some frames where that number approaches a half inch or more. That would be noticeable in the case of a relatively small frame and/or narrow mat widths.
That feature is quite new on the site. I don't understand it either since it's depicted as if it were a frame in 3D (shadow sides and all), instead of just a "crop" of the mat size. I hope he tweaks it, 'cause it's confusing.
Heh, it's like all software -- never done, and always adding new features whether necessary or not. Those changes were new to me too, and I haven't really digested them, but I think the frame "moulding" gets wider, as it might with larger overlaps. I agree a nice simple solid color would be more than sufficient -- but then, I usually use small profile gallery frames and may be biased.
On the old version he had a comment about allowing for overlap, the main problem being you could adjust the "mount" dimensions to make the allowance, but then you had to add the overlap to position the image from the mat edge when putting things together. The new version appears to do all that for you.