i've speculated about this 54789 times.
in the books there are subtle clues that ghosts can hold objects. so if they can, why couldn't they help fight in this war? throw solid books on peoples heads etc. if can go against them or for them.
plus i was reading something on.. some website... and it said that the grey lady would be back in the next books.. so i think the ghosts will play a part in the war.
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"if you let them make you; they'll make
you paper mache; at a distance you're
strong; until the wind comes; then you
crumble and blow away"