I have done text conversations for NPCs but only if I already have the conversation typed out in advance and can cut and past it over... which is almost never. Occasionally I will think up some, in my opinion, particularly good dialogue and have it handy, if it fits, to past it over, but even then I usually just say it, as usually what I have written the players have skewed slightly so reading it verbatim doesn't make sense and parts of it must be improved. I am simply not a fast enough typer to do all entire conversation off the cuff in text, unless I absolutely have to, without feeling like I am making the players wait for me to finish and really killing the games speed.