When a mistake is that glaring, I have a hard time taking the author seriously, which means their message is muddled in the morphological mess.
While I am not averse to some grammar-nazi-ing (I even like it sometimes), I think it’s important to remember that not everyone is text-aligned. You will get a lot of agreement in a medium that relies on text to show status and so on.
So, go for it — just don’t complain too much when speech oriented people nitpick your public speaking skills, or graphic designers trash your website or bean counters criticise your lack of quants for every single thing or extreme extroverts complain that you are on your computer when you should be mingling. Everyone has strengths and weaknesses – the internet allows the illusion that strong writing == permission to look down on everyone else.
It’s a big mistake in any case to have a hard time taking anyone seriously for anything really. When the chips are down and I need a good decision on my program, I’ll take my poor-spelling-but-evaluation-genius colleague over your good grammar any day. On the other hand, that same colleague will not be allowed to write my blog posts. 🙂