I agree with you; insofar as there is a 'market' * (e.g. job market) in which particular linguistic performances (e.g. use of standard spelling) have a value, then we'd be doing our students a disservice not to support them develop such competencies (I nearly wrote 'literacies'!).
I guess the thing that bugs me a bit is the 'literacy in the singular' attitude that's intolerant of language forms that deviate from that standard - e.g. txtspk - even when those language forms are creative and rich and absolutely appropriate to context. This article - I h8 txt msgs: How texting is wrecking our language exemplifies this attitude.
* see Bourdieu
The constitution of a linguistic market creates the conditions for an objective competition in and through which the legitimate competence can function as linguistic capital, producing a profit of distinction on the occasion of each social exchange.(Bourdieu 1992: 55)References
Bourdieu, P. (1992). Language and Symbolic Power. Cambridge: Polity Press.