Hey, what a great discussion.
I've been of the opinion from the beginning that JK wanted to make the war in her books seem like real war like Johnny said. And in real war people die for stupid, stupid, arbitrary reasons. If you're serving in Iraq, you're best friend can die tomorrow by stepping on a land mine. No moral. No thematic purpose. Except for maybe the ubertheme: war is arbitrary and horrible because people you care for die for no reason at all.
Johnny, I take exception to your distinction between fiction and the newspaper because there have to be some elements of realism in a work -- even one this fantastic -- for us to care at all. Otherwise it's just Looney Tunes -- Fred gets killed but comes back for the next episode.
JK has gone on the record (see Lunchy's post) saying that Harry's owl's death represented the end of his childhood and the beginning of his adulthood. But ultimately, that death and the death of all the other good guys that died in book seven were, I believe, meant to be arbitrary and superficially meaningless, because that is how JK perceives war, and that is how she wants us to perceive it.
In fact, there is something and fresh about a fantasy story that purposefully makes you regret the killing of a number of good people for no apparent reason, rather than loading every (rare death) of a friend of the protagonist with thematic and plot significance (see, e.g., every piece of crap David Edding's ever commited to paper).
Moreover, the whole arbitrary thing is catching on.
*SPOILER ALERT*
Remember in Serenity when Joss Whedon kills off the pilot dude suddenly, without any apparent plot or thematic reason, and somewhat arbitrarily (the cockpit glass could withstand space particles but not a balliste -- give me a break). I had major flashbacks to that scene when I read that Tonks and Lupin were dead.
Even further, the good guys seem to suffer a lot more casualities than the bad guys. Another theme: idealism has its costs. If the Order had been as willing to use unforgiveable curses from the beginning (and yes I know Harry, McGonnagal and Mrs. Weasley all used them at one or more points -- but they definitely did not do it as often as say, Bellatrix) they probably could have lowered the causualty rates on their side. But they didn't for moral reasons, and paid the price in blood.
So, in sum, even the meaningless deaths in Hallows arguably contribute to a larger theme that JK has repeatedly espoused: war is hell and good people die apparently meaningless deaths. Now JK's stuff is pulp, it's not literature. She's not making any point with all these deaths that Voltaire didn't make in Candide. But she deserves kudos for introducing a relatively profound point in the context of a pulp work that happens to be the most popular series ever.