"A thousand words, called out through the ages..."
Taken from a line from the lyrics to the English version of Thousand Words by Jade, these words are a pretty good representation of what exactly is happening with the more recent RPGs. Vocal tracks are becoming more popular than ever, especially in RPGs. The earliest instance I can think of is One Winged Angel, the music played during the final battle with Sephiroth in Final Fantasy VII. I mean, it was just background chanting, but it was still quite a milestone for me, to hear something other than bleeps and bloops in the middle of a fight.
The trend continued with Final Fantasy, with Eyes on Me being the main theme from Final Fantasy VIII, Melodies of Life as the theme for Final Fantasy IX, Suteki Da Ne (Isn't it Beautiful) for Final Fantasy X, and then Thousand Words for Final Fantasy X-2. Of course, there are plenty of other RPGs out there with vocal tracks (Xenosaga had Kokoro, for instance), these are just some of the more popular ones.
This, really, is only natural. As games become more and more realistic, they begin to transcend the barrier between movies and games that have graphical capabilites on par with movies. As such, it makes more sense to advance the music from the beeps of yesteryear to fully orchestrated vocal tracks.
Could you imagine wandering around in Crystalis while having someone singing in the background? I can't even fathom how weird that would be.
Music has always been an integral part of any RPG, and adding vocal tracks adds another layer of difficulty in providing flow and immersion. Chances are you wouldn't want to have singing while wandering around Vvardenfell or the Dragon Quest 8 overworld. Same goes for battle music. There will always be a need for orchestral music (or beeps :) ) in order to provide a subtle background noise to keep you entertained.
We need to have vocal tracks as the games advance in technology. The games are telling a story, and song is yet another medium with which the developer can tell it. When Suteki Da Ne played during the ending theme of FFX, it just added to the satisfaction of completing the game, due to the incredible combination of the orchestral melody and Rikki's wondrous voice.
I find nothing wrong with adding vocal tracks. If anything, like I've explained, they add to the feel of the game and the overall experience. The only thing developers have to worry about is where to place the song. Once that's settled, then the type of music needs to be decided. Anything can be used. We've seen things from lounge music, to even heavy metal (Otherworld, in FFX).
I'm excited to see where the music world takes us in the future.
Suteki Da Ne?