Consider also that they are at war... it is irrational to think that all the main charaters will make it through. Even if there is not some higher purpose to their death I think the needless death of a character could act to portray the cost of war. She has been known to make a poitical statement or two in her books at times.
Also... I am pretty sure more then 2 major characters will die. She said that she killed of 2 characters she wasn't initially planning on killing off and one got a reprieve. SO I am thinking that in her to do list of charater deaths (could be 10 for all we know)... one got taken off and two more got added on.
I would also be ok with Harry dying in the end... if she did it right. When he looked into the mirror of Erised in book 1 his deepest hearts desire was to be with his family... it could make for a beautifully heartbreaking ending. That being said I want to fab 4 to live (Harry, Ron, Hermione and Ginny). The rest I am ok with having to die (even if they are amongst my favorite charaters).
My top 2 choice on who will bite it (of the good guys) is Hagrid and either Arther or Percy (hopefully proving why he is a Gryffindore by saving the life of someone in his family)