ExploderWizard
Hero
The thing that seems odd to me about not doing some amount of optimizing is that adventurers are killers. Their job is putting themselves into clearly dangerous situations over and over again. From an RP situation I can't see how surviving fights can't be a reasonable priority for a PC. I don't see what's unreasonable about a PC who makes every decision about personal improvement based on what will make them the best killer. And people who have a party role of getting into melee who don't optimize to survive melee would, in a realistic world, die sooner and more often.
I'm not trying to defend players who are jerks, or who optimize around questionable rules, but if there's straightforward and rule-compliant way to design a tank that makes the PC more effective in combat then the professional tank PC will rationally want to make those decisions. If that means that every tank plays the same class with the same build, then in-game either everyone who wants to be a professional tank will learn that build, or every adventuring party will try to recruit people with that build.
I think the idea extends to other party roles, too. If a PC is the party's sneaky thief then they should have a build optimized for being a sneaky thief. In-game sneaks who don't use that build will either get themselves or their party killed. I think the argument is even stronger for party faces or healers, who will get their entire party killed if they're not good.
This is MMO logic and may or may not be true in a tabletop game. Sure, its possible to run a campaign of just a series of fights, and in such a game having a party of optimized fighters is helpful.
Many campaigns do not fit that mold.