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."
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.