Of course audible voices make it easier to get immersed. It should. In reality we talk face to face and there`s no magic text that appears, so I would of course prefer audible speech. It`s natural.
The only reaosn I`m a little against voiced dialogue is because of the obvious loss of detail we get in dialogue branches. Even today with the latest games they still don`t match the detail of story and dialogue of something like Morrowind. And as long as you are prepared to READ, and the story is GOOD, you WILL become immersed..
But it is not impossible at all to become totally immersed in a text only fps rpg, it really isn`t. Anyone who says that has not played games where they don`t always expect everything to be voiced.
When they have full voiced dialogue with the depth of Morrowind or Baldur`s gate or even Planescape Torment, then we can say there`s no need for text games.
I have been playing games since Space Invaders. The first game I ever played on a PC was the text adventure game "Adventure" back in 1984 or 1985 or so. I've played a number of text adventures - and yes, they can be immersive, because just like books they are all text. First or Third Person RPGs become immersive through entirely different means, and having conversations take place through text breaks that immersion. They're visually so real that it is a jarring disconnect when this quite-real-looking person walks up to me and "converses" with me via a text window - it really is the equivalent of speaking to somebody through written notes.
Text-only games are, as I've said, as poor a comparison as books because both use entirely different means to immerse the player/reader than do other games, and text is used to convey everything.