The simplest answer I can give is that spelling and grammar are basically learned facts, sometimes they are single facts (how to spell a single word), sometimes they are rules ("i" before "e"... which ironically is a mostly incorrect rule, but hey it's just an example). As such, why is Person A considered less intelligent for not knowing how to spell "there" vs. "their" than Person B who doesn't know the date on which Hitler shot himself? Or Person C who doesn't know the triple point of water? Or Person D who never learned what the whole P vs. NP thing is about?
The second reason is, in my opinion, the most important: even if you simplify people down to two categories, "people who have fully grasped all spelling/grammar" and "people who have done their best but still fail to get it right", the only way to say that the second group is less intelligent than the first is to massively simplify intelligence, the same way IQ tests do.
If you're a mathematical genius but have terrible spelling, poor social skills, and basically no skills in anything outside maths, are you intelligent? What about if you're a literary giant whose best-sellers are praised by snobby critics and working class teenagers alike, but who couldn't square 15 without a calculator, are you intelligent?
Here's a quote I like:
"But there has always been something opaque about I.Q. In the first place, there’s no consensus about what intelligence is. Some people think intelligence is the ability to adapt to an environment, others that capacity to think abstractly, and so on."
I think I've probably rambled on quite enough, but will just finish on a subjective note. I score high on IQ tests, I'm quick with numbers, I (until getting bored and dropping out of school) got high marks on exams without studying, yet I can think of many people I consider more intelligent than myself who couldn't say what I have just said about myself. Think about it yourself - I think it's unlikely (though possible of course) that you're the most intelligent person you know, even by your own judgement, and of those you rank above yourself, I doubt they all excel in every single area that could be used to judge intelligence.
With the general "i before e"... Is being more often true than false enough to make it a valid "rule"? Using your numbers it's correct ~60% of the time, with my dictionary it's correct ~76% of the time:
But even at 3/4, not sure you can call that a good "rule", if it's wrong that often. But of course, that could be swung subjectively if the most used words follow the rule and the least used words break it... which I imagine is the case, since people do still use this rule, but I'm not sure of an easy way to argue the point.
Edit:
I suppose one way would be to assume that british-english-small (with 50083 words compared to insane's 638286) is comprised of the five thousand most used words (which I assume is roughly the logic?)...
Small shows that "i before e" is correct 88% of the time, and "cei not cie" is correct ~27%.
So in fact, limiting to most common words does make both percentages go up, the "except after c" is still much more often wrong than right, and the general "i before e" is... perhaps now high enough to call a good rule? Not sure.
Grammar and vocabulary is a learned social construct with peculiarities associated with the social group the grammar is derived from. What is correct in one social group may be wrong in another. For example, my first sentence is incorrect in some social milieus as it finishes with a preposition.
Demonstrating "correct" grammar merely demonstrates memorization of expected social norms ('communicate as the group does') for the group that one is a member of -- or wishes to be a member of. "Incorrect" grammar is not a global quality as all grammar and vocabulary is local with respect to group membership, geography, language exposure, temporal presence, and other factors. In addition there are certain idioms and informal speech present in specific social groups that may not conform to even that own group's peculiar grammar and lexicon!
Grammar and vocabulary are often a type of shibboleth used to demonstrate group membership or to discern group membership of an individual you are evaluating. Quite often pedantic demonstrations of grammarian prescriptivism is merely attempted to assert their social norm on another person of a different social group membership and are often uninformed as to the derivations of the rules they are attempting to impose. In other words, outside of pedagogic concerns, it's a dominance demonstration typical of primates and should generally be considered as nothing more.
The second reason is, in my opinion, the most important: even if you simplify people down to two categories, "people who have fully grasped all spelling/grammar" and "people who have done their best but still fail to get it right", the only way to say that the second group is less intelligent than the first is to massively simplify intelligence, the same way IQ tests do.
If you're a mathematical genius but have terrible spelling, poor social skills, and basically no skills in anything outside maths, are you intelligent? What about if you're a literary giant whose best-sellers are praised by snobby critics and working class teenagers alike, but who couldn't square 15 without a calculator, are you intelligent?
Here's a quote I like:
"But there has always been something opaque about I.Q. In the first place, there’s no consensus about what intelligence is. Some people think intelligence is the ability to adapt to an environment, others that capacity to think abstractly, and so on."
That's taken from http://www.nytimes.com/2007/09/14/opinion/14brooks.html?_r=1 which follows nicely on from what I'm saying.
I think I've probably rambled on quite enough, but will just finish on a subjective note. I score high on IQ tests, I'm quick with numbers, I (until getting bored and dropping out of school) got high marks on exams without studying, yet I can think of many people I consider more intelligent than myself who couldn't say what I have just said about myself. Think about it yourself - I think it's unlikely (though possible of course) that you're the most intelligent person you know, even by your own judgement, and of those you rank above yourself, I doubt they all excel in every single area that could be used to judge intelligence.