One important point about the DN5 is that the gap between adjacent topbars is one beespace. DN1 and such like with the narrow top bars give more space than beespace - hence the bracing.
Secondly, Hoffman sidebars stop frames slapping or swinging when the hive is moved, or even when brood boxes are lifted. Why crush bees unnecessarily?
The standards are not perfect. Following them slavishly results in dimensional and practical problems, and they have not been updated since the British Standard status was allowed to expire without renewal at least 25 years ago (IIRC).
Another example is that the external dimensions given in the standards generally assume a certain thickness of timber; these days timber is planed below the nominal measurements hence 18mm timber may well be even thinner when measured, and of course 18mm is already thinner than 3/4". There ain't many mm in a beespace, so each extra one you end up with is important!
DN5 is definitely an improvement over standard frames. Worth the extra given that you expect 10-15 years of service from them.