"I think it's good to kindly point them out (it could help a poster land a great job by improving their covering letter). Why should those of us whose grammar is generally good have to learn how to decipher what is being written in a post?"
InadvertentlyBrilliant it's not kind if people find it rude, patronising or hurtful (and personally I would find it to be all three of those things).
We aren't applying for jobs, nor are you our employer. We didn't ask for your corrections. We are chatting informally on a forum. It's arrogant and condescending to think this is an environment where people should welcome (and feel grateful for!) your unasked for criticism.
And it's very rare indeed surely that a post is indecipherable? If it is, perhaps you could click on something else - you don't have to decipher anything or reply to anything here which is not up to the standard you expect 
I recognise that you feel some people require your corrections because they aren't intelligent or educated enough. But (this is a genuine question and I'm genuinely interested in your answer to this) how can you tell whether someone is saying there instead of their due to lack of education or whether they have (eg) dyslexia, brain injury, side effect of meds, processing issues etc?
You really could offend someone in that case (I fall into this category and I would be offended). You can't tell from my incorrect use of "their" whether I didn't listen in English lessons or my brain can't retrieve the right word due to a disability.
Is your need to "help someone get a great job"
or make something easier for you to read, worth upsetting someone by pointing out and criticising their incorrect grammar (which is caused by a disability)? You mention kindness, it would be kinder to say nothing. If you absolutely really felt a terrible need to correct someone, you could PM someone I suppose - but then of course you wouldn't be showing everyone else on the thread how superior you are?
If I could kindly point something you to you, it would be you can be kind or you can be right.