This is a tag to the end of 13x17 and a bit to 5x04 also. I saw someone mention this on a tumblr post, and while now I can't find it again, this little idea stuck with me and here we are. Plaid shirts for the win! Hope you guys enjoy, comments are always appreciated.
Don't own SPN.
The apocalypse world is a war-zone, both literally and figuratively. It's a bombed out wasteland, Dean was able to see that from their first and very brief encounter there. It was cloudy, ashy, desolate, and even the air fought to get in and out of his lungs. But he was a soldier, and soldiers went into war-zones all the time, and this was no different. He had a job to do, and it was to bring mom and Jack back, and he'd be damned if he didn't go.
Of course, he isn't exactly in the correct clothing to be headed into a war-zone, but not that it really matters. Sure, he could have used the excuse that there wasn't enough time to put on something more suitable, or that it was constricting, but in all honesty, maybe it's because it just wouldn't have helped. Or maybe it's because he's seen what he looks like as a solider, alternate universe or time travel or Zachariah messing with his head or what, but he doesn't like it.
Dean Winchester walks straight into a war-zone with a plaid shirt and jacket on his back, boots on his feet, and bag of weapons on his shoulder. There's nothing else. No fancy toys, no black tactical suit, no bullet proof vests, no thigh weapon holsters, no army green camo coverage. He heads into the wasteland in the same attire he wears around the bunker, or to take out a vampire, or to go out and have a drink.
Ketch, of course, is all suited up, not that it'll help if something truly awful decides to hunt them down and make them go poof. Maybe for Ketch, his outfit provides him with a false sense of security, and Dean's not about to question him on it. Or maybe Ketch really doesn't know what they're walking into, and Dean couldn't care less other than the fact that the man's his only back up unless something goes wrong and Sam has to come in. It's just the fact that a similar outfit wouldn't do Dean himself any good, for a multitude of reasons, the primary reason being what happened just before the apocalypse.
In 2014, at Camp Chitaqua, Dean saw firsthand what he looked like in a soldier's garb. It wasn't traditional, of course, but post-apocalyptic soldier outfits didn't include helmets and camouflage. His future self, wearing dark green faded clothes, a gun on his hip, a gun on his thigh, a continual serious, unfazed and unmoving look on his face, and the weight of the world quite literally on his shoulders, was not someone Dean aspired to be. Hell, it was someone he changed the past for so that he could avoid becoming.
Sure, his future self had seemingly gotten the job done, but not without fault, and it had killed both him and his friends in the end. He had, however, done it all without empathy and humanity, the very things that Dean continued to fight for when he had made it back to his time.
Seeing Bobby, well, not their Bobby, the war-zone version, decked out in his soldier gear, had brought Dean right back to that memory all those years ago. If he was ever going to participate in a war in a wasteland, he was going in with his humanity and his intentions intact. What that meant, in a split second decision, was one that left him more vulnerable physically, but more intact psychologically.
So yeah, Dean freaking Winchester walks into war with a blue jacket and a white plaid shirt and a bag of guns to keep him protected. But that's all he's ever really needed to get the job done the way that sits right with him. He wore plaid shirts as a kid, when his father tasked him with protecting his brother, his first mission as an unofficial soldier, and the getup hasn't needed to be changed since. He probably wore a plaid shirt when Lucifer made an appearance, or when he talked down the Darkness from ending all creation. He's not about to change his battle gear for another end of the world. Apparently, he had once before, and it hadn't ended well. And where possible world ending events are concerned, Dean's keen to learn from mistakes, even his own, to maintain some semblance of humanity in a place where it possibly no longer even exists.