The main reason I would like text only is because of the sure depth of dialogue and iformation you would get.
Unfortunately it's also unnatural, and not conducive to the portrayal of a living, breathing world. It's jarring and disruptive, which they're working to get away from as evidenced by how time keeps moving in dialog now and you're no longer frozen in place. It'd also prevent things like NPCs talking to NPCs... if it's text-based dialog, there's no unintrusive way for you to be able to "listen" in. Made worse by the fact that different people read at different speeds and will go at their own pace, which wreaks havoc on the intended pacing of the exchange (a book would have much more to work with to get across the intended pace of a scene than a video game with text-based dialog).
Text works where text makes sense. In-world books, notes, signs, etc. Using text in place of what should logically be heard (ie, speech), when dealing with an audio/video medium like video games, is backwards thinking. It'd be like going back to silent movies because you could skip having to film actors speaking their lines, thus requiring fewer takes and less overall production time, and just interject still frames with written dialog on them. You could even have much more in-depth dialog that way because more written dialog doesn't take as much time as more spoken dialog, and you'd have a greater pool of actors since you didn't have to worry about what they sound like.
Personally, I found Morrowind's dialog to be a good reason to move toward actual spoken lines. Morrowind's dialog was completely illogical if you tried to imagine the characters actually saying it. It was so descriptive and in-depth in places, that it felt less like a character talking to me and more like quest/reward descriptions put under the guise of "dialog". With Oblivion's dialog, the conciseness made it much more believable that it was something a character would actually say, which, along with the emotion put into the speech, made the character that much more real.