[chaseme]

I have to agree to the OP if all anime use

informal or improper Japanese as well as slang,

.
I dont think that all anime uses informal or improper and slang japanese so i have to partly disaggree

and i think some people knows what to absorb and what to ignore.
if they what to know formal japanese they would probably learn from formal anime/jdrama characters and not from characters that is regarded as idiot/stupid characters like naruto

i agree on the part that the best way to learn the language

If you really want to learn proper Japanese, take classes, hang around people that speak the language, and study from lesson books.

i just think exposure to anime/jdramas do helps
and i think watching undubbed ones are better, since we learn our first native languange unsub

***i dont really speak japanese but when i was learning english, films and cartoons are great exposure for me