I think that a bigger point against spell check is that: Given a context, we are extremely good at auto-correcting. IM, being a conversatory mode of communication, defines and keeps re-defining context clearly. Also, we humans are good at re-iterating mutual feelings thru conversations, thanks to paraphernilia like :) , lol , ha ha, :( , :D , :P ....
Ironically, there are hundereds of spell check plug-ins readily available for every IM client.
PS: Here's a poem that passes thru a spell check. It makes good sense to humans too. But its a total nonsense to an English teacher.
Eye halve a spelling chequer,
It came with my pea sea,
It plainly marques four my revue
Miss steaks eye kin knot sea.
Eye strike a key and type a word
And weight four it two say
Weather eye am wrong oar write
It shows me strait a weigh.
As soon as a mist ache is maid
It nose bee fore two long
And eye can put the error rite
Its rarely ever wrong.
Eye have run this poem threw it
I'm shore your pleased two no
Its letter perfect in it's weigh,
My chequer tolled me sew.