The only pisser is thinking that people are so two-dimensional so as to only ever be one thing, or in this case, Class.
I have had a number or really interesting online discussions with TS over the past month or so as she is part of the Skype B/X game and like me, she really enjoys discussing gaming.
Her comment is one I hear often in relation to the limited class selection, lack of multiclassing rules, and/or race as class in B/X and I would agree wholeheartedly with her if B/X was a simulation. However, B/X is a glorified wargame. And I revel in this fact. B/X does not try to represent any type of reality and verisimilitude is not something that is strived for.
In real life I am much more than an ex-investment banker (at least I really hope so). Just like in real life an infantry battalion is more than just a bunch of guys with guns and an armoured battalion is more than a bunch of tanks. However, in most wargames they are portrayed as singular types.
Just like different types of units in wargames, Fighters, Thieves, Clerics and Magic-users are designed to provide different tactical options. A party made up of five fighters is going to have different tactics than a party made up of two fighters, a cleric and two magic-users.
In B/X, are all fighters the same? No and Yes. Ability scores and roleplaying are the only real ways to distinguish one fighter from another. As I mentioned in an example I gave in the B/X Ability Scores post, "A DM can reasonably determine if a character can swim based on his background."
But besides these arbitrary differences, yes all fighters are the same. And they have specifically been designed as such.