I don't mind them, I suppose. I saw them play on the big stage at a music festival once and I enjoyed their set. It was a particularly memorable experience because the stage was raised so high (as they are at festivals) and Este was wearing a short dress ... so they did their coordinated moves and of course the crowd cheered in unison every time they'd see up her dress, especially when it was on the big screen. Regardless, it was one of the better sets on the big stage, for sure.
I don't like the idea of 6+ (is that how many?) collaborative music videos and a co-lead role, only to have them score the thing. It would seem too insular at that point. But hey, I guess their late-70s/80s pop influence would kind of suit the period. However, their core sound is so pop-oriented and major key uppity that I just can't see there being enough versatility to provide a whole soundtrack for a movie. I don't see any evidence of extensive knowledge of period music either. But I guess it depends on the type of movie. I still don't see a full "score" happening.
I think it would be more likely that a new track or two from Haim would be written and recorded for the movie -- if anything -- and the rest of the score would be overseen by someone else.