I think we need to distinguish between two very important types of corrections: 1. somebody makes a grammatical or spelling mistake that possibly confuses those that may be reading the post and 2. somebody makes a typo (e.g. smoething - thanks Cicero) and doesn't have time to catch the dotted, red line. There are plenty of times the grammar police will come on here correcting the typos that don't need to be corrected in order to understand the post. This goes back to the concept of "adding to the discussion". If a correction adds to the discussion by possibly clarifying something to the other readers, then by all means correct away. But too many times the corrections are, to a degree, pointless.
I understand the arguments that this shows the destruction of our English language, but that was busted up long ago by the allowance of "slang".
But a problem does arise with only correcting what needs to be corrected in order to clarify a post: if the post needs clarification, how does the correcter know if there interpretation is correct? This can only be corrected by asking the original poster their original intent. With this said, we can certainly view all "correction posts" of the unnecessary variety, unless it is a clarification question directed at the original poster.
My personal opinion is that a public forum is no place to "teach" somebody about proper English by condescendingly telling them where they are wrong. If you really want to "teach" somebody, correct them in a positive manner, like a real teacher would do. I personally don't know of any grade school teacher who would yell out the corrections condescendingly to their student in the middle of class.
If you don't agree with any of this, ok, difference of opinion, but you cannot claim being a "teacher" if condescension is the only method of teaching. Call it what it is: being a jerk.