Why do people think because you can spell and write really well you’re intelligent?
I'm useless at both. Does that make me stupid?
If I asked you to change the brakes on your car could you do it?
If not does that make you stupid?
Being unable to read and write properly leads people to believe you are stupid as speaking and writing the queens english is taught to you from a very young age and, you do tend to get a lot of practice. What they should actually say is that you are inarticulate or uneducated (not that either sound better). That is very different from being stupid.
This is not aimed at you Obladeo, but you asked the question. Trying desperately not to tar everyone with the same brush or make over-generalisations, most people would say they are not thick or stupid. And they'd be right. But being able to change the brakes on a car requires a different skill set from reading or writing. Some skills are innate, others are learnt. It is our ability to learn and retain information on a broad range of topics that most people would use as a measuring stick for intelligence.
So the fact of the matter is that intelligence is a difficult subject to quantify and hence why the IQ test was setup. Guess what guys and gals, there is no spelling test on that. So, make of it what you will. To those people who cannot read and write properly, its never too late to learn or use a spell and grammar checker, then no one will ever know.