The two-way mirror - a cruel useless plot device

[Maria]

No: HPFGUIDX 163671

I am usually the last person to critizise JK Rowling, but upon
rereading OOTP, a thought has been lingering on my mind.

The two-way mirror is not used at all during OOTP, except to make
Harry feel even worse for his actions. He got himself, all his friends
and the Order members, in great danger in his attempt to save Sirius
from the Department of Mysteries. There were other things he could
have done to notify the Order about his dream, like telling Snape, as
he realized too late. He  discovers the mirror only too late, and
feels even worse if that can be. But I wonder... why? Why did Rowling
write the mirror into the story? It's not needed to make him feel
regrets or guilt, he already does that. It's not needed for any other
reason so far. And, if it appears again the Deathly Hallows, she could
have introduced the device into the story by other means, like part of
the things Harry inherited from Sirius.

So, why did Rowling need to make things even worse for Harry? It's not
like she needed filler material to make the book longer, if anything
it could have used a trim here and there. I think it was a bit cruel
to make Harry realize the mirror was there despite everything else he
went through.

Maria