The core framework ensures that your character will fill a useful and meaningful role in combat without trying to hard. This is valuable when you are trying to generate a positive first impression on new players. New players (and new DMs) are the target audience when the rules discuss roles and party optimization.
Optimization, however, is not required to play the game. Indeed, you can still grow unoptimized characters in 4E, but you have to work at it. From a design perspective, it makes far more sense to require rules mastery to grow an unoptimized character than it does to grow an optimized one.
But even beyond an individual character's growth, party optimization is not required; party synergy is. An optimized party (however it is defined) can fail if they do not work together, and a non-optimized party can succeed if they do. Profound, I know, but I feel that people equate optimization with synergy and subsequently feel rebuffed by 4E if they do not want to optimize their character or party. You don't have to optimize either, but you do have to work together (well, unless you're into those dog-eat-dog campaigns).
You're non-optimized party will have to address situations differently than an optimized one, and may have to work a bit harder or be more creative to achieve success in some situations. But if that wasn't the case, exactly what meaning would "optimized" have?
And as a final thought, none of this touches on the options and opportunities characters and parties have for development outside of combat, something that roles, (most) powers and 90% of the PHB don't address. There is a whole world outside of how your character fights where you can plant your flag and define your character with little regard for what the rest of the party is doing.
