User blog comment:Serza5/One Vocaloid, 5 Languages?/@comment-24785637-20140819174802/@comment-3076833-20140820163256

The extra phonetics, if anything, at least ensures that if there are people daft enough to buy a Vocaloid outside the language they want to use it in, has some of the sounds they need to pull off the voice. But otherwise I like to see it being more for songs which have those odd lyrics in another language so that it isn't completely Engrished.

MAIKA's English is nice with the only real downside being the heavy Spanish accent which comes from the fact the sounds are primarily Spanish. But MAIKA has so far been the only multi-phoneme bank to prove that she can still hold quality in it. Sonika's case being fairly obvious as she lacks much quality to begin with. In SeeU's case the extra phonemes, at best, can sound like  Circus-P using Japanese banks to imitate English (i'm not a big fan but I'll happily give him extra credit for not making Japanese voicebanks sound like complete Engris).

To sum up i'm all for multi-phonetic banks but even I will admit that there is a lot of issues with them and it's going to be a while before some sort of perfection can be made with them.