The main issue I have with it is file size. Admittedly compression techniques are better than during the N64 days, but there's only a limited amount of space in a NDS card. Many games that use full voice acting use disk space larger than the Largest known NDS card. (Exception is given to Guitar Hero/Band Hero.)
Voice acting actually complicates things, at the moment, For example in B/W, when N speaks to you. He will references your lead Pokémon by it's name or species.
The main issue is that some Dialogue that uses variables which is made from the player choices.
In PBR, the latest Pokémon game with voice acting. The Narrator actually said each Pokémon 5 times to reflect the 5 different "moods." (That's 5 times 493 Pokémon, making 2,465 files just naming Pokémon) Most "lines" (despite being cut up) are repeated, in different "moods".
Your Name is almost impossible to record just in the off chance it's used. Many games that use Allow you to this NEVER refer to you by your chosen name. In Fully voiced Skyrim, you're called Dragonborn or Dovakin often. Same goes for your first name in Mass Effect Series. Sometimes they refer to the player character without gender identifiers.
Text to speech in Japanese version if you force phonetic symbols only, which is doable, but messes up existing custom Pokémon names. Take a look at Zelda: 4 Swords Adventure's Navi Trackers, or not as you can't play it in the non-japanese versions. Anyway, I've found a video of someone using the name Ritsu.
http://www.youtube.com/watch?v=aVm4QqhfSVo&feature=related Tetra will mention the name a few times, first time at 29 seconds in to confirm the name. This game allows 4 characters (or Links) to play at the same time Tetra runs a commentary with guests popping in. Well
Zelda does have a cute sneeze.
Goodish news is that Text to Speech engines are getting better all the time, and almost human or can be used in cell phones. I believe that assuming that it's programmed well, Vocaloid is almost human.. And they brought a nearly complete "engrish" Voicaloid to the PSP. Although I think it sing about the need to
feed on a tree frog. Seriously Snake really knows know how to infiltrate the Voicaloids:
http://youtu.be/N64IDscE5NI