Klaus does care about bloodlines, at the very least because of the spark. A Heterodyne is more important than a Merlot or a von Zinzer. The reason why he used his son as a heir was probably that he needed the strongest spark he could get. And this could also have brought him to choose him as his own vessel.

In effect, important nobles often are sparks, and autonomous rulers I remember all are (Gil, Klaus, Albia, Zeetha's mom, that other Queen, the Iron Sheik, Voltaire, Colette; not sure about the UnderParisian kings).

Even if he wanted to think outside the box as far as non-sparks are concerned, he still lives in a world where lots of people care about bloodline, as evidenced by the Storm King conspiracies. So he has to take it in account.