just jumping in to say there is a wonderful Chuck Jones exhibit at the museum of the moving image in NYC right now, and it is set to travel to a few other locations. it addresses the music and the incorporation of the shorts in features. according to the exhibit, the shorts were initially simply targeting a move-going audience. one has to consider how different that was back then - there were features, news reels, and shorts. it was not like going to a movie today, to see a singular film, but rather as an encompassing source of entertainment. i also believe that they were used frequently by the same studios (i.e. a WB film was likely to have WB short). The exhibit gave the impression that the immediate impact was a love of the characters. Bugs and co. and Merrie Melodies were very well received from the onset and the production at Warner Brothers went from something they were trying to something highly marketable for them very quickly.

edit - also, they functioned as war propaganda pretty early on. im unaware of the impact/success of it though. but as i mentioned, the characters were well liked so the pro-america/anti-nazi message was probably easily received as well.

Generalised follow-up: most history of animation that I'm aware of is focussed on animator biographies and the medium's technological development. When it is discussed as an art form, I feel like it's treated as if it exists in its own bubble. Any good sources discussing its relation to other art media? Big picture discussions on animation, if you will?