I have seen claims that a softer spine will increase the edge holding over a fully hardened blade with the same edge heat treatment. The reason being is that the softer spine will act to absorb the shock on really heavy use and thus prevent crack formation and migration along the edge. I never tested this, but have spoke to engineers who have and they agree, however they were working with much larger "blades" and didn't think that the effect would be significant on a hand held knife.
In regards to modern steels, the point has been made correctly that there are current steels that have a very high impact toughness and ductility even at rather high hardness like 58+. This means that you can run them at full hardness and not have to worry about premature breakage. This means that you don't have to deal with the drawback of differential tempering which is that you significantly weaken the blade. This is not a small effect either. Roughly, since strength is proportional to hardness and the contribution to the overall strength is actually maximum near the spine, you can see a strength drop of 25-50%, depending on how much you soften and to what extent .
-Cliff