This HBR article by Kyle Wiens, takes a hard line on people with poor grammar:
On the face of it, my zero tolerance approach to grammar errors might seem a little unfair. After all, grammar has nothing to do with job performance, or creativity, or intelligence, right?
Wrong. If it takes someone more than 20 years to notice how to properly use "it's," then that's not a learning curve I'm comfortable with. So, even in this hyper-competitive market, I will pass on a great programmer who cannot write.
I would agree. In fact, I used to agree with this view.
Unfortunately, I can't, because I've met some extremely intelligent people who consistently mis-spell things. For example, Bob Leung, my cofounder on Woobius, is one flagrant such case.
Before I worked with Bob, I too held the view that grammar and intelligence were generally correlated. But then Bob posed a bit of a quandary. Here was someone clearly very smart, extremely talented, extremely driven, with great attention to detail, and who is completely incapable of writing a single paragraph of text without at least one or two mistakes in it.
Since then, I've revised my judgement, and I would urge Kyle to revise his too. If you're hiring writers or programmers, then certainly, attention to written detail is paramount.
However, for many other functions, like sales and design, while attention to detail is important, it doesn't always manifest itself as good grammar.
So my advice is: do not judge people solely by their grammar - it is just one factor amongst many.
If you read this far, you should follow my RSS feed here.