Although I thought it was a mistake on Lucas' part to make some of his characters speak that way, mainly because some people were offended by it, I myself think it's a more realistic touch that they do and should talk with an accent. It makes sense that someone who is foreign to another language would have an accent. And since most speak Basic in their galaxy (which is our own world's English), who's to say the Neimodians don't have an Asian sounding language, too? Besides, Watto was from Tatooine and one of their main languages is supposed to be Huttese. Hence the accent. It's funny how nobody mentioned Bib Fortuna and his accent in ROTJ, but they jumped all over Watto's. Having these characters speak basic English would have been safe, and probably boring, too. But if Lucas would have gone with a new alien language instead, I'm sure it would have made the story more interesting and the Neimoidian characters would have been appreciated better as villains.