For IRC or chat, it doesn't make sense to use capitalization or most punctuation. The beginning of the line serves as a delimiter between messages; capitalization is redundant in this capacity. Likewise, the end of the line serves a similar purpose, which makes periods (generally) redundant. The addition of a period in this medium adds extra emphasis to the message, which is often interpreted as aggressiveness (hence this article).
Also, it is unfair to associate lack of capitalization with poor grammar and spelling. Poor grammar is associated with lack of education. Lack of capitalization should not be! You probably were not implying an association, but many others I have spoken to over the years have. (For some reason, I've had a lot of conversations about this stuff...)
That being said, there are plenty of reason to eschew writing best practices, and you have to be pragmatic about it (for your own sanity, if nothing else). Have a hand injury? Skip the shift key once in a while if it saves you some pain. You're trading readability for speed/comfort/whatever, but depending on the situation that might be the better decision.