You know it doesn't work like that, so why bring it up?
I bring it up because, from where I sit, it does work like that. The system is linear - the difference between 10 and 14 is exactly the same as the difference between 14 and 18. If the first difference is small, then so is the second. In games with exploding dice, or dice pools, you can get some funny spots in the math where you see a sudden increase or decrease in effectiveness. But D&D's math doesn't have any magical phase transitions or discontinuities, unless your GM is doing something odd when setting DCs.
And, as I said, if you're playing across the gamut of levels, in the long run the stat bonus gets left behind (barring magical enhancement, which also tends to make the stat you started with less relevant).
Anyhow, there are sometimes other ways of getting to much the same point, at least with a good group.
I'm playing in a Star Wars Saga Edition campaign, in which every character has at least some levels of Jedi. "Use the Force" is a Charisma-based skill. So, as you can imagine, everyone's taken pretty solid Charisma scores.
Except me. I chose to take a Charisma of 10. No bonus whatsoever. You'd figure, then, that I might as well just keep my mouth shut during social scenes, because everyone but me has Charisma bonuses, and some of the others also have training in social skills, while I do not.
I still manage to be persuasive in social scenes. How?
By having higher Intelligence and Wisdom scores than the other characters, and being trained in knowledge and perception skills. My character is better at reading people than the other characters, and is more likely to know relevant information. The end result is that when he opens his mouth, he impresses with what he says, not with how he says it.