Basically, in the essay, what Fussell is doing is pointing at the people who disagree with him, and saying "Your opinions are invalid because your experiences haven't biased you to agree with me."
Which is a view we see echoed over and over and over in sociopolitical debate today. Fussell's own "If you'd spent nearly a year getting shot at in Europe, YOU wouldn't care about killing civilians to save your own ass!" becomes "If your city had a suitcase nuke hidden in it, YOU wouldn't care that we had to tape a car battery to some terrorist's scrotum to save your own ass!"
Or "If you'd actually been in New Orleans and gotten shot at by looters, YOU wouldn't complain about FEMA!"
Or "If your family had been murdered, YOU wouldn't oppose the death penalty!"
Or "If you'd gotten your job stolen away from you by some Mexican, YOU'D change your tune about immigration!"
First-hand experience of a complicated or controversial issue can be useful, because it gives a "street-level" view of how the issue looks to and affects people in general. It provides tangible and concrete evidence of just what the stakes are.
But this is a double-edged sword-- the intellectual benefit of concrete personal experience can be overwhelmed by the emotional impact of it. If you're not careful, it can turn the debate over the issue into a matter of pure feelings rather than a balance between reason and emotion. When the debate becomes all about emotions, then the most emotionally manipulative are the ones who frame and control the debate.
("Here's my story about losing my leg to a German shell on D-Day. But I didn't get my leg blown off just so some punk could say whatever he wanted to about the President! That's why everyone should stand united behind whatever the government does!" "...but I don't agree with--" "Look! Look at this stump where my leg used to be!! See how much pain and agony I went through to save YOUR freedom! Aren't you GRATEFUL?!" "Well, yes, but--" "Oh, you hippies are all the same! You say you're SO caring and SO open-minded! You'll cry millions of tears over some poor endangered owl or some homo who can't get a job--but you don't CARE about the HEROES who SUFFERED and DIED for YOU!! If it wasn't for me, you'd be in a NAZI DEATH CAMP!! WHY do you HATE FREEDOM?!")