I was wondering if they would perhaps cause people to appreciate the English Vocaloids more. Many people gain false impressions of English Vocaloids because they pick up faults in their own language that they do not in a foreign language. This causes a biased and faulty opinion.
However, if the Vocaloids they adore gain English banks, perhaps they will see and adjust their judgement. I hope so.
Worst case senario: We have a case of hypocrisy; fans (you know the type I mean) preferring the new Japanese-English banks because of their designs and familiarity.
Also: People being against the creation of English banks because (although they may not realise this) it shatters their innocent 'perfect sound' persception of Vocaloids that they gained through ignorance of the language.
Personally, hearing the Engrish produced by Japanese-only banks made me really appreciate the original Engloids in my early days of Vocaloid. So this made me wonder.