Too many people will spill there emotions into a discussion with sub standard spelling and grammar which makes it very difficult for others to understand them, which leads to inevitable misunderstandings.
I have real trouble imagining that a good point made with substitution erros is somehow less critical to the discussion than a poor point that doesn’t confuse it’s/its.
How would you expect someone taking on a programming challenge of say, an operating system, without the basic understanding of boolean logic to fair?
More importantly, I think it's fair to ask that people learn basic grammar, and if we don't take it upon ourselves to insist on seeing correct grammar, then that will never happen. That's why I think being a so-called grammar nazi is important. Now, being a dick about it is a different matter, but somehow people think you're a dick regardless if you try to correct their spelling/grammar, so there's only so much that can be done there.
That said, there are definitely some levels of pedantry that can be varied. For example, I tend not to be too put off by it's/its. There/their/they're, however, is annoying for me, because the words themselves have such monumentally different meanings. Same deal with fair/fare. Then/than is annoying to me because I actually pronounce them (subtly) differently, so as I'm internally reading the words, it throws me even more.
Overall, I'm really agreeing with you -- I don't think there is serious potential for misunderstandings. And indeed, a good point made with grammatical mistakes is more important than a correctly-made bad one. But a good point made in `proper' English is, I think, better than a good point made in `bad' English.
Another good Muphry's law example.