Some historical perspective:
According to one estimate, if Homo sapiens had existed for 24 hours, writing only came along after 11 p.m. Thus spoken language is fundamental, while written language is an artifice.
In that light, John McWhorter defends informal speech in email and texting:
Keyboard technology, allowing us to produce and receive written communication with unprecedented speed, allows something hitherto unknown to humanity: written conversation. In this sense, they are not "writing" in the sense we are accustomed to. They are fingered speech.
A sense that e-mail and texting are "poor writing" is analogous, then, to one that the Rolling Stones produce "bad music" because they don’t use violas. Note that one cannot speak capital letters or punctuation. If we accept e-mail and texting as a new way of talking, then their casualness with matters of case and commas is not only expected but unexceptionable.
On the other end of the spectrum, Lewis Lapham rants against Internet writing:
The strength of language doesn’t consist in its capacity to pin things down or sort things out. "Word work,” Toni Morrison said in Stockholm, "is sublime because it is generative," its felicity in its reach toward the ineffable.