Why do you think even games tagged as "visual novels" have usually a full voice-over? The text without voice lacks emotion for one. Yes, I know you can imagine all that in your head but it is not the same.
In the games like Dragon Age you can just feel the tension and emotions of the characters with voice acting alone. I sure doubt if that game would be half as fun as it was without it. Also the occasional song sung by the characters could be a nice touch. I still remember the Beer Song from Bard's Tale
...or in DA, that Leliana song was quite nice too.
