MN is written communication, so errors that aren't apparent in speech will be rather glaring here, in cold hard type.
People who post aggressively and who insist they are superior deserve to be taken down a peg or two if they make obvious mistakes in their posts.
Correcting mistakes can help people to learn... if everyone ignores errors, how will they ever realise, especially as teachers seem to believe that they should only correct a few errors per page and leave the others in case it discourages children? No wonder we have two generations of people where many struggle with grammar and spelling.
On the other hand, it can be rude, people are posting here in a relaxed manner, or in a hurry, or while doing something else as well, or using their phones...
The dyslexia excuse doesn't ring true, btw, all the dyslexics I know work quite hard at getting written communication right. I bet people with dyslexia are more likely to own and use dictionaries than the general population...
Ultimately I don't think it's as simple as either 'pointing out mistakes is always rude' or 'using incorrect spelling or grammar is always rude (to the reader)'. Sometimes it's worth knowing you've made a mistake, sometimes it's rude to point it out.
(I tend only to point things out very politely when someone is making a mistake repeatedly that might embarrass them in real life - can't think of an example but if they are using a phrase that they've obviously never seen written down - OR when someone is being pig-headed and nasty but has made loads of stupid mistakes.)