That is only if everyone has the icon turned on.
Though on reflection an icon that one has to turn on and off in their profile would not be a good idea. A better idea would be to have an icon that can be turned on and off for each individual post. That way the person correcting the mistake can have the icon turned off, forbidding anyone to correct any of his mistakes for that particular post.
An icon also does not force people to correct your spelling errors, it only lets everyone know that you will not get annoyed if anyone happens to do so.
This would explicitly let people know that a poster does not want his spelling or grammar to be corrected in anyway. Getting rid of a huge annoyance for some people. It would also allow people who do correct spelling and grammar to do so without fear of any ridicule or negative stigma.
This feature still does not allow people to go from thread to thread only correcting spelling errors. I have seen someone being warned for too many off-topic jokes that they made in GQ. That same warning can apply for someone who has nothing to add but spelling corrections. Corrections would be better off made along with some useful reply to the topic.
The hope is that everyone making spelling corrections now would shift their attention to the people that actually want the corrections made. Since people are inevitable going to make corrections in spelling, why not shift their efforts towards people who value their advice?
Or we can have this feature strictly for the OP only. This way the OP is telling everyone that his topic can shift to a meaningful discussion of grammar if anyone wishes to do so.