Have you ever tried making a great recipe, but don't have alI the ingredients? You can't expect it to realistically be great, maybe average, right? Well try making a film without a music supervisor, it may come out a little cold, bitter, too much sugar but not enough spice...
A film trying to cut cost by supervising music in-house, might as well light a match to the movie. No one can replace the job of music supervisors, mainly because it's not everyone’s cup of tea.
Great supervisors wear many hats but have a common goal of translating emotions & sentiments throughout the film into music. I can visually see a scene and automatically ask myself, what instruments are needed, does this music fit the demographic and era, or would an original song work better? This skill-set becomes second nature to experienced supervisors over a period of time. I've seen films without one, and you see strange things like a Barbra Streisand song playing in a dance battle or sticking a Beethoven song in a movie for the hell of it because it's royalty free. The madness needs to stop! But selecting the music is only half the war. Let's pretend you actually have a budget like 99% of films, and they want a masterpiece. Obviously, if you're a small film, you can't ask for Bruno Mars songs with MC Hammer budget. But there's this thing called music licensing, and if not handled correctly, you could be sued handsomely.
For example Michael Jackson's movie “This Is It”, was notably sued for overuse of the license to Michael Jackson’s song catalog. What a shame for Michael Jackson's own movie to be sued by Michael Jackson music. Smh. But seriously, you have to understand there are many types of licenses, and if you're not experienced to understand what that license holds, there's not going to be a happy ending.
Film makers want their films to be memorable, captivating, and even revolutionary to people who watch it. That same desire needs to be applied to the music. Not having a music supervisor, is like not giving your project a pulse. Films are what we watch, music is what we feel!