This question already has an answer here:
I love the ease of correcting misspellings, typos, and grammatical errors in SO. Certain errors catch my eye and annoy me - and the wikiness of SO is a great blessing; I don't have to live with the errors, I can simply fix them. But...
It can be useful to know how sloppy the OP is. A question that reads, "plz hlep me fix this" can be safely ignored - safely, in the sense that if it wasn't worth the OP's time to express him/herself, it may not be worth our time to help out with answers. It can also be useful, say, when it's clear the OP isn't a native English speaker - we can calibrate our answers to make them more readable.
And a healthy edit obscures all that useful information. So, where's the balance? Personally, I'm on the side of "Fix it!", but I can certainly see the case for leaving it alone.