Bilbo was wearing chain mail...
Anyways, you can't just throw classes at an old story and assume that it counters his point. His point is correct that a seasoned warrior would make optimal choices if the choices were available to him and he had knowledge of how they worked. However this only works to a certain extent, for instance a sword and shield defending expert may never have the idea to take a pact from a demon to get extra hp when he drops someone. It's not in his knowledge set. However you better believe if he finds a set of plate on the ground he is going to use it.
It goes both ways, the rp backs up the mechanics, and the mechanics back up the rp. Both are necessary and good parts of the game.
One also has to assume that there are simply things that matter which are beneath the level of granularity that is covered by game rules. Things that would be too tedious and boring to keep track of in the rules. Plate armor is hard to maintain, uncomfortable to wear, hot, etc. It might be a little bit better in the middle of a fight, but then again, maybe not. Depends on whether you want to move fast or resist a lot of blows.
4e was good in this regard, there was always a trade off, always another way to get some better defenses, and always some OTHER defense that might serve better than a point or two of AC. It kind of subs in for the unwritten things that matter, so in the end the guy wearing plate armor in that game DOES have a bit better AC than a guy wearing leather armor, but its not some huge gulf, unless one character is simply built ignoring all defenses. If the rogue and the paladin each follow sensible practices, they'll each have a pretty good AC, and where the paladin's AC is better, he'll lack in some other respect like speed or reflex or something.
The other thing is, if you have a game where narrative aspects are important and play a role in the game, then many characters simply acquire the abilities that they find a need for at the time and place they get them, or that are easily available to them. Its not like CHARACTERS have access to the rule book to know that the sword they chose isn't as good as some other hypothetical sword. Again, this is an area that 4e was nice about, since you could retrain most choices later on. If you don't have a bastard sword RIGHT NOW, but you DO have a +2 magical longsword, well, its better to specialize in that than gain proficiency in the more effective weapon that you don't even own. Next month you may find a nice bastard sword and retrain into that.
Obviously all these considerations can go out the window if you're making a 'blank sheet' character and just want to be a perfect fighting machine or whatnot. There's nothing wrong with it, go to town, and at that point a 1% better at something might as well be taken, sure! OTOH I really doubt the characters can tell the difference at such a fine level. IN GAME there are probably endless arguments about whether its more ideal to learn to hit harder or parry more effectively. Just like in the real world each will have its proponents, even though it may be possible to prove one to be universally superior if you can perform a sophisticated enough analysis. There's always going to be different levels of skill in each, and its not like CHARACTERS know what level they are either, so they're never going to be exactly sure what beats what, because they only have what they observe, noisy data. So non-optimum choices to the player, knowing the mechanics, can't be attributed to stupidity of the character.