Clinton:
Take a look at these two statement of yours:
It's not just about being understood. You're running a business. There will be some readers of your posts who judge you (and your business) on these things. While the odd mistake is often excused, a cavalier attitude may not be treated so lightly.
While professionals, from accountants to business consultants, can be severely impacted if they come across as semi-literate, I accept that plumbers, car mechanics and baby-sitters may not be so judged on the quality of their written communication. I wouldn't judge them harshly either.
Note that I've italicised the important bits. Do you still not see what I'm getting at here? Or I am getting it wrong? Steve? Steve? Help? (but don't, if you're just gonna back Clinton up LOL
)
With CVs - that's a whole different ball game. Yes, there are people who think their CV is brilliant.
I actually know of one guy, who had/has a CV, and when I saw it, riddled with spelling mistakes and errors, so I went full throttle on it, and helped sorted it for him. Whether he still uses that CV or not, I've no idea. And no, I don't know if it helped him get a job or not.
I also think CVs are complicated beasts, and what I mean by that is that what one person thinks is a good CV, someone else might not. However, spelling and grammar is still the same.
Whether it does or doesn't help a candidate is also debatable. Some employers don't pay too much attention to the spelling/grammar 'quality' of a CV, others do. I am sure that there are plenty of employers out there, that have taken on someone, based on a CV that DID have errors in it, but the employer didn't notice!
The fact that you don't get called for interview, can be due to many many reasons, some of which are often total b*ll*cks!
Customers who decide to go with a competitor is usually due to cost, or customer service, or other reasonable factors. I wouldn't necessarily decide NOT to use a service or buy a product, purely because they happen to have made a spelling/grammatical error somewhere.
BTW, I'm not offended or upset by any means that you keep spotting my mistakes. It'll take a LOT more than that to offend/upset me!
P.S. If you don't have friendly banter, things can become boring 
Barney - I agree / am familiar with everything you've said there (I think) 