There are limitations on how 'realistic' our RPG worlds can be ('realistic' meaning 'makes sense, internally consistent', not 'The Real World of Planet Earth'). We might not want to examine how wearing armour makes you harder to hit, or that you can choose whether or not your sword strike was actually just a knockout blow with the flat of the blade
after you already resolved the attack and damage of a killing blow...
Sure, there is a willing suspension of disbelief that is essential to get into character for RPGs which just isn't there when playing chess. "What would a rook do in this situation? I know
my rook is a sucker for a pretty face, so I won't take my opponent's queen; I'll ask her on a date instead!"
But it is possible to mess with things in order to service 'balanced' so much that it makes the world so unrealistic that you can't get into it any more, and just treat it like a board game. If I was looking for
that then I'd give up and play 4E.

*ducks for cover*
It is also possible to move things too far in the 'realistic' direction, and everybody has to play farmers who never go further than seven miles from their village....
Getting the recipe right is crucial, and different tastes mean that the exact balance point varies from table to table. For me, the idea that every person's stats are equal takes things too far away from realistic.
The DM puts us in situations where we imagine that we are our PC and do what our PC would do. In order to make a sensible decision, the world must be sensible. Game mechanics reflect things in the game world, so although game constructs like 'class' and 'level' cannot be known to the creatures in the game world, their equivalents can be (he's the toughest hombre north, south, east AND west of The Pecos!).
What we want to avoid is unrealistic assumptions on how the world works based on player knowledge of the peculiarities of the game mechanics. When we make decisions for our PCs then our PCs should make those decisions based on things they can know. When the BBEG demands the party to send out its most powerful member for a duel to the death, it does not make sense for the party to reply, "No can do! We are all built on 27 points!" Partly because they are talking about game mechanics they cannot possibly know.
If they said, "Don't be stupid; it's a well known law of nature that everyone is equal in total ability if they have had the same experiences", because the game world actually works that way, then that game world is so unrealistic that it would take me out of the role-playing and just leave me with a board game.
If fifty people you know were to have their Str/Dex/Con/Int/Wis/Cha objectively rated, there is no expectation that they would all add up to the same point-buy total. The world doesn't work that way. 'Realistic' worlds do not work that way (except for CloneWorld: The Apocalypse). Imagine a world where you only need one size of clothes or everyone got the exact same SAT score.
We all have varying amounts of tolerance for that kind of unreality, and varying needs for the PCs to be identical in order to be 'fair'.