I agree in some cases, but simple words like Nani? Ano? Arigatou etc. are 
pretty easy to understand. And even the more difficult ones are also if used
in a good way. For example, Kun-chan's lemon series uses a LOT of Japanese

words

to describe *ahem* yet it is pretty obvious what they mean. AND, as a

bonus, you

learn a little Japanese.

imho, who cares about using those words?  i think it's...well, bad form, in
a way.
we speak english, you can live without japanese in your story, therefore why not

I guess it's 'bad form' to write in English if someone's going to read it in
America, too?

just stick to english?  i think this ranks up there wtih using multiple
punctuations
(!!!) or (???) or even (?!).  not terribly horrible, but bad form anyways.

Which English would you like? English English or American English? I guess
it's 'bad form' to write in 'English English' if you're going to read it
in America since that's another language.

All my stories have Japanese in them somewhere. If you don't like it, you
can lump it. I'm not going to stop writing some stuff in Japanese and I am
not going to write in American just to make it 'good form' for you, either.
So... Beeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeedah! :p

Kun-chan...