It depends. If I was 100% sure I wanted a bareknuckle I would buy a 54 since I already know that is the best sized bareknuckle for me. I wouldn't be able to straddle a 56, and would have excessive seat post showing on a 52 and have to run excessive spacers and/or a high rise stem to get the bars where I want them.

But what if I have 3 or 4 frames I have narrowed down, and looking for which one is the best fit. Stack and reach is a good way to compare them. Along comparing to my current bike which is dialed in pretty well. At least the best way for me, being in a situation where I won't be able to test ride the frame first.