As a counterpoint, think of most action and comedy films made during the 80s. As soon as you hear those synths and drum machines, suddenly you are pulled out of the film and you are aware that you are watching a product of the 80s, not a timeless action classic. The most egregious example of this I can think of is the film Ladyhawke, ostensibly a film set during the middle ages. If Richard Donner had chosen to use a traditional orchestral score, his film would probably had more of the timeless fairy tale element he was after. Instead, as soon as the action scenes start and the pop Alan Parson's score starts playing, you are immediately pulled out of the middle ages and plopped squarely into the 1980s.
For another example, look at two major SF films made during the 1970s - Logan's Run and Star Wars. As brilliant as Jerry Goldsmith's score is for Logan's Run, it can't escape that disco 70s vibe due to the heavy use of now dated-sounding synths (of course, the "70's shopping mall" setting of the film doesn't help either ;) On the other hand, John William's score for Star Wars has a timeless quality mainly because he deliberately avoided using the then trendy "sci-fi" synth elements.
Regarding the use of existing music or songs in a film, if you want to use a popular song that you think is crucial to the film's success, make sure you have determined whether or not you can afford the rights to the song or if the artist is even open to letting their song be used in a motion picture. Don't expect that a distributor will take care of that problem if they decide to pick up your film. The chances of that happening are extremely slim. My advice - before you put a song in your film that becomes crucial to the storyline, make sure that the song is available and you can afford it! If you are not sure about the availability or affordability of the song, find some up and coming acts to contribute a song that conveys much of the same tone or emotion that you are after. There are TONS of bands looking for exposure, and they are easy to find.
Just stuff to think about :)