if you watch modern movies like the lord of the rings series, they also use odd dialect. it adds ambiance to the film and id say modern dubs are going for the ambiance angle on venues that occur in a different time or story universe.
the settings for most anime are not current day normal earth therefore the language helps create that feel. think of something like the lord of the rings series, they do not speak normal english because it would ruin the story.
you have to note that anime went through developmental phases and the reasons for odd dubs through those periods seems to have changed.
something from the sixties like astro boy or even the early eighties like techno police has a very stilted english dub and as it appears to have been translated by non native english speakers then read by voice actors (reference?), the translation appears to use phrase-book snippets from sources that were thirty or forty years out of date when the translations were made. On the other hand some american cartoons such as minute mouse and courageous kat also use the same odd phrases and stilted delivery. whether it was for effect or to mimic the up and coming japanese animations i do not know. of relevance is the fact that several japanese films from the sixties and seventies also used the same odd idioms and stilted delivery on their english dubs. some of the martial arts related ones come to mind and have become cult films due to the justaposition with the delivery not matching the action and the almost irresistible urge to repeat the line out loud for comedic effect.
it was a successful formula for that time period and success doesnt always know why it does what it does, it only knows it has to.
these dubs are clearly odd because of how they were translated and i believe the stilted delivery was intentional to match the level of realism in the animation or create interest. later anime with smoother animation used more natural deliveries although as pointed out, the dialect is not colloquial and that is for effect.
i am familiar with these phrase books. on trips to asia or hosting visitors ive commented on the odd phrases the locals used trying to speak english and in several cases they produced the phrase books and showed me how it was "correct" even though they did not know it was hilariously outdated.
modern english dubs can be at least as good if not an improvement on the original japanese. i cite FLCL as a prime example. over the top voice acting full of expression and the use of current day phrases. in the case of FLCL the odd dialect has been replaced with odd intonations and japanese idioms which have no direct translation are replaced with english idioms that convey similar levels of emotion. the goal is still achieved: the story is told with feeling and set and setting are clearly other-world.
note that an anime set in the 'real' world such as 'the graveyard of the fireflies' has a smooth dub with normal dialect. no need to provide the setting of an otherworld venue.
on a side note regarding liberties taken with dubs, sailor moon was the first major work that i know of where the practice of trying to follow the japanese script exactly with a dub became less important than trying to tell a coherent story. in the case of sailor moon, the story presented to the american audience is much different than the one told in the japanese language release. the japanese language release detailed her attempts to find a boy and had significant sexual overtones. her magic girl quests were almost a nuisance hindering her from completing her dating task.
such an emphasis on dating and sexuality for a young teen was deemed inappropriate for the christian-conservative north american audience and the script was amazingly re-written telling an almost entirely different story while using almost all the original video. thats more than just an odd dialect to create ambiance, its an entirely different script.
id love to see interviews regarding this with some of the voice actors and directors from the sixties and seventies while they are still around. if anyone knows of any, please post some links!
c dos. cd dos run. run dos run! (sorry i migrated here from the computer side of stackexchange)