What's optimal hardness for a kitchen knife blade?
Unfortunately, even hardness is a trade-off, all other things (like maintenance, appearance, balance, thickness, and so on) being equal:

Harder


Pro - Holds edge longer
Con - More difficult to hone on a steel
Con - More brittle, so more likely to chip


Softer


Con - Doesn't hold edge as long
Pro - Easier to hone on a steel to an extremely sharp edge
Pro - Less likely to chip
Con - More likely to get little dings and dents


I think the manufacturer's indicate the type of alloy they use to help indicate the quality of their product, as opposed to more generic knives.   Things aren't always equal, though--in choosing a knife, I would not consider this fact one of the more important ones compared to how it feels in your hand, the thickness of the blade, how much maintenance it requires (for example, so-called carbon steel knives need more loving care).