I never bought the idea that a silent protagonist helps immerse you in the game. I always thought "I'm controlling this character. It's not me jumping over fireballs or dodging bullets, it's the character. It doesn't bother me if the character dies since he/she will come back." If I play as a particular character, then beyond any gameplay advantages that character has over others, I'm playing as them for their looks. If Yoshi flew off the edge of Rainbow Road, it's Yoshi who flew off the edge, not me. First-person view where you get no hint of the character existing helps a little, but I still know who I'm playing as.
I think the reason the characters are still silent is 1. it was that way in the previous games due to lack of technology, let's not change that, 2. we can make some silent jokes out of it or use it as an excuse to work on body gestures, and 3. because Zelda CD-I happens if we include voices.
I'm fine with silent protagonists because they find a way to make it work. Mario's fun to watch when he's going through motions at breakneck speed to explain something (besides, it'd be more universal and have more of an impact than reading a description of the same thing). I can't imagine Gordon Freeman spouting off Duke Nukem one-liners as he's dealing with aliens, but who knows, if I had played Duke Nukem I might also like the chatter. If you're going to include voices, please have a reason for it. While Deer Avenger 2 is a horrible game, the deer main character does shout out hints on what to do next and how to play better, so at the cost of breaking the fourth wall it's a clever way of HELPING the player. Don't include voices for the heck of it, or else you're better off having stayed silent in the first place.
So in other words, I'd like Mario to have a voice, but I don't want them to include a voice just because they can. If Mario has interesting stuff to say or he can contribute something useful in a cutscene (a GOOD cutscene, none of this Super Mario Sunshine airplane stuff), then go ahead. Just as long as the voice is tolerable, he's had an awfully strong accent with his one-liners.
Regardless, silent or not, does NOTHING to help or hinder immersion, because I'm never immersed in the first place. Any immersion I'd get is in watching the characters hopefully get safely through whatever level they're in.
Sidenote: In Final Fantasy X, there was an impromptu fight with a boss right when I didn't need it and which ended up being a lot more difficult than I anticipated. So I just barely make it through that, the boss flies up into the heavens, and Tidus yells "and STAY up there!". I could say I was immersed there because Tidus was yelling exactly what I was thinking. And you totally needed the voice for it to have the right effect. I get some immersion if the character thinks like I do, that's the only way I can relate. That's what I want to see more of.