Additionally, quotes from The Game Mechanics constitute one designer's opinion and not official policy. Perhaps the reason this "rule" never appears in a Wizards of the Coast book is because there wasn't universal agreement in R&D
Except that it is the explanation of the d20M approach to classes.
From
"Classes and Basics: Countdown to Modern"
"The idea was to create a system that was capable of handling characters with very few real skills (first level characters), as well as providing a flexible system of progression that allows characters to develop in a virtually unlimited number of directions. The great example Chris gave was Buffy the Vampire Slayer -- the main characters started as "ordinary" high school students, but gained and developed new skills, talents, and abilities as the story progressed.
Bill pointed out that the Basic Class system will cross every modern genre. And, when you get a look at the rules, you'll see why. It's almost unbelievable how well this core game mechanic lends itself to doing just about anything. When I asked how the system was developed, Bill said that he wanted to really take advantage of the versatility of the d20 System. The rules for d20 Modern were arrived at very organically. Chris added that he thought the system was "brilliant."
He's right. When you think about it, any character you have can essentially be broken down to an emphasis on a particular ability (or combination). My wizard is all about Intelligence, and my old thief put all of his ability score bonuses into Dexterity. It really makes sense to build a character creation system based on those fundamentals. The interesting bit is that because the Basic Classes are so pure, character concepts can be taken in vastly different directions. You could want a soldier type who's an expert with firearms -- that's a Fast Hero. Your pal might want his soldier type to be able to take a beating and keep going -- that'll be a Tough Hero.
That's what's really cool about the d20 Modern system -- the rules are all about what your characters can do, not what they can't. Your character concept will shape your PC more than anything, and the choices you make when assigning class levels will just help to reinforce what you're trying to do.
You want a soldier who's really good with guns and can take a couple hits? Well, then you're looking at multiclassing. (Here comes a good bit.)
In d20 Modern, there's no penalty for multiclassing. In fact, it's highly encouraged. Here's a quote from the book:
'The classes in d20 Modern represent the wide range of skills and knowledge that people in the modern world have access to. Each class level represents an area of expertise that a character trains in. Therefore, multiclassing can be done freely and without restriction.'
Chris pointed out that by 4th or 5th level, just about every character will have multiclassed at least once, or picked up an advanced class. (Advanced Classes are similar to prestige classes in that they're more focused on a particular set of skills or abilities. You'll get a look at the advanced classes in a few months -- just hang on.)"
From
Advanced Classes
"Most characters are likely to dabble around with multiclassing in two or more of the basic classes. Really, that's what the system is set up for. The truly impressive thing about the d20 Modern Roleplaying Game is how it's been designed, from top to bottom, to allow maximum flexibility so you can play the character you want to play. It's all about options."
From
Behind d20 Modern
"Bill's character concept for d20 Modern was quite elegant: The game needed only six classes, each based on one of the game's six abilities (Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charima). This gave us the Strong, Fast, Tough, Smart, Dedicated, and Charismatic hero archetypes. Using these six basic classes, players could build heroic characters of all kinds, from college students to private eyes to Marines."
The six attributes lack flavor as character classes. Spycraft had the right idea.
Well, some people like that lack of flavor. I, personally, don't like the spycraft classes. I know others that feel the same way which is why we don't play Spycraft. Back to Basics might be an improvement over the Spycraft class, but I haven't checked it out since there are other aspects of Spycraft that I don't like in comparison to third party d20M supplements (e.g., Spycraft's treatment of Martial Arts in comparison to Blood and Fists).
If you posit that the three physical attribute classes make for fun combat encounters and the three others make for fun skill challenges then you only need nine Modern 4th Edition classes. Strong/Smart, Tough/Wise, Fast/Charisma, Strong/Wise, Tough/Charisma, Fast/Smart, Strong/Charisma, Tough/Smart, and Fast/Wise.
Only if you think every character needs a combat role. I personally do not. Not every character is about combat and not every person wants to run a modern game where every character, let alone any at all, are trained/experienced combatants.