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The entire point of the way romance was done in book 6 was as a demonstration of the childishness of the relationship between Hermione and Ron. It will only be through growing up and having a mature relationship that Ron or Hermione will be at their best. Until then, their relationship is a distraction. Harry and Ginny (no matter whether you "like" the relationship or not) is a mature relationship. Harry does not yet realize that he loves Ginny. However, we can say that he does by his self-less treatment towards her. This relationship will play a part, and needs to play a part, in the last book. I would not at all say that Jo is bad at writing relationships because it would imply that she is not good at natural characterization, which is, in my opinion, one of her strongest abilities. It is her life-like characters that makes Harry Potter so vivid and real. And because of this ability, she understands how the characters react to each other, including romance and falling in love.
I do think the whole plot of HBP was about maturation, romantic and otherwise. And in this sense, the romances reflect, as I've said elsewhere, the tensions and difficulties in the other aspects of the story, even as they are something of a diversion from it. There is a tremendous need for maturing between Ron and Hermoine, both for the sake of their relationship and for their individual self-discovery processes. Harry and Ginny have had more of that kind of processing foisted on them, at least Ginny has, but more needs to be done if there is time. But all this happens against a background of Harry learning what his loyalty to DD really means (that it is not DD helping him be special, but DD training him to fight LV serious; that choices really are the backbone of the events that befall us; that people are not always what you want or expect them to be, etc.). And this is against the background of the title: the young Snape helping Harry, the young Snape having become the actual potions master, the young Snape taking on (what I hope and believe) was the terrifying responsibility of killing DD for the bigger cause. And maybe the book was about Harry becoming himself the Half Blood Prince at last, ready to fight Lord V.