One argument which may or not work depending on the venue is that colloquial, abbreviated, misspelled or grammatically incorrect English is hard to understand for non-native English speakers.
Bad grammar makes it hard to parse sentences. Where's the subject, where's the verb? What word is this word a complement of? A native speaker can often understand non-standard constructions intuitively, but a non-native speaker will often need to consciously analyze the sentence.
English spelling is only loosely tied to its pronunciation. Non-native speakers are often more familiar with written English. Natives often use phonetic spelling, but a non-native might not even realize that their and there are pronounced identically: they're just vaguely-similar words.
Misspellings make it impossible to look up a word in a dictionary.
For native readers, the damage isn't as bad. But even for them, if one person writes in txtspik, l33t or whatever, every reader has to do the effort of deciphering. That's one person's wannabe-coolness or ignorance to weigh against the work performed by the many readers. Making others do more work gratuitously is selfish.