One of the nice things about Classic or 1E (or even 2E) is that it's pretty easy to just say something like "These creatures are an offshoot tribe called 'Pyrogoblins'. They're normal goblins except that each can breath fire at one target 1/day for 1d6, save for half." You just give them 1 bump on the xp table and you're done.
I'm not saying that can't be done with 3E. But the monsters in 3E seem "built" according to the rules, and it's ambiguous as to what constitutes an increase in CR, etc. When players are struggling to squeeze every bonus and synergy out of the system, to give a monster an arbitrary bonus may seem to some to be outside of the DM's power!
I'd much rather just write "Boss Hobgoblin: AC 4, HD 4, HP 28, D 1d8+3."