Please take what follows with a pinch of salt: as the Americans say YMMV.
With regard to the sizing question for those who are around 5′ 10″/5′ 11″, and are trying to decide between 57cm and 54cm, I think it may be a case not of one size being wrong and the other being right, but rather that it depends.
The key factor in the choice is probably the effective top tube, and how that compares with what you currently ride/would like/think is best for you.
That said, I think you might prefer the 57cm if you like having bars fairly high, possibly even level with – or no more than maybe around 50mm below – the saddle height; in other words more of a relaxed traditional touring style position. This might particularly suit those who have less flexibility. Indeed I think it would make a superb flat barred touring bike.
Conversely, all other things being equal (e.g. height of spacers used), I think 54cm will allow a much more aggressive position and style of riding, including off road, which might appeal if you want to closely replicate your position on a road race bike, especially if you prefer lower bar height. This will probably also put more weight on the front wheel, which I think will improve handling, especially off road.
One thing I am not sure about is toe clip clearance/overlap when mudguards are fitted to the 54cm, depending upon the tyre size and guard-tyre gap. Overlap is not normally an issue on the road, but a strike might be more likely when making a low speed manoeuvre off road. I don’t think this will be a problem with the 57cm. I think Somafunk has a 54cm to which he has fittted guards, so he should be better able to comment on this.
For what it’s worth, I think a lot of the changes over the years in road bar height has been driven by fashion, rather than what is best for performance. I recall reading that the top tube of Pantani’s Bianchi on which he won the Tour and Giro sloped upwards from the seat tube, rather than down from the head tube, so that the bars could be positioned relatively high with few or no spacers, which meant he could comfortably use the drops in his distinctive attacking style in the mountains.