First of all, I think there was an argument made for some of the previous Harry Potter movies to get an oscar on technical merits and depending on the year/movie they might have been robbed.
But after that, the ones that people really care about (directing, writing, acting), none of the films deserve a win in these categories.
I have seen every single HP movie, multiple time, (my wife is a big fan) and while some of them are enjoyable (and yes, some of them are down right cringe worthy), none of them stand out as being oscar worthy.
One of the limitations (and possibily, one of the good things about the series actually) is the enormous amount of screen time given to the three child (well young adult now) lead actors. From a purists stand point with the books, I really like that the series stuck with this approach, even though from a movie perspective, they are all fairly rotten actors.
This is the limitation the series has worked against - it is,ultimately, a movie about children, starring children, for children. Children are not always the best performers, and frankly when doing a long series of movies like these ones, they were kind of "stuck" with the crop of kids they picked up when they were bloody eight years old. You find a casting director that can spot an eight year old that will be able to turn in an oscar worthy performance in 5-10 years, and you'll have found someone who has very bloody likely signed a pack with the devil.
Anyway, all that being said, Harry Potter doesn't need to win an oscar to justify your enjoyment of the films, or your care of the characters, or source material, or to prove how good the books are or any other such nonsense. In fact, I would be somewhat cynical if the series was awarded some sort of prize as a nod. At this point it would almost be like saying, "Here's an award for having the... guts?? will??? determination??? to cash in on a crazy popular phenomina and make several by the books movies for a large fan base over the last ten years!"
It's not very inspiring if you think of it that way, is it??