I've corrected people's SPaG on here (and had my arse handed to me for it!).
Its not so much that I find it annoying, or cannot understand the post, more that I find poor literacy tends to devalue the point that is being made.
It just seems a shame that someone with a perfectly valid viewpoint or opinion makes it very difficult to take them seriously due to their poor SPaG.
There was a thread just the other day were the OP's actual issue got totally lost in her use of malapropisms such as 'battered an eyelid'.
Isn't it better to have comedy errors pointed out to you on a forum like this so you can avoid them when you're at work, or with someone upon whom you need to make a good impression ?
I guess I see it in the same light as letting a stranger know that she's accidentally tucked her skirt into her knickers or something - surely people would rather know than walk around making a fool of themselves?
With me, it definitely comes from a good and helpful place, although I don't generally comment these days as I know how badly it goes down.