I think it depends on what the film is going for. As others mentioned, a film like Schindler’s List, or for a more recent example, 12 Years a Slave, doesn’t need to be rewatchable. In fact, I would go as far as to say that fact that I don’t want to rewatch them to be a sign of their greatness. Those are well made films covering very heavy historical topics. I knew it would be moving to watch them, and having experienced that so fully the first time, it would be difficult to do it again. If I felt I missed some aspect of the story or emotional impact and NEEDED to watch it again, I think that actually detracts from it’s greatness.
There’s also other types of films that aren’t meant to just drag us emotionally through the mud, but rather take us on a wonderful journey, the type of journey we want to take again and again. A great example of this would be Shawshank Redemption, it’s a drama, but I love the characters, I love their interaction, and I leave at the end of the film feeling better.
What about non-dramas? Well, a good comedy ought to be rewatchable, it may not be as funny the second or third time, but it needs to have that staying power. Sometimes a comedy is very topical, and it ages poorly as a result, but even those can be great if they can make that topical humor transcend whatever the particular reference is to still be funny. And maybe a shock joke will lose some punch, but some jokes get funnier and funnier every time I see/hear them, and they’re endlessly quotable.
Even popcorn flicks can be great, maybe not worthy-of-an-Oscar great, but the type of film that has a lasting impact on our culture. Of all types of films, these MUST be rewatchable. Consider Jurassic Park, it’s greatness is precisely why Jurassic World made so much money last year, and that film is rewatchable. Or the upcoming sequel to Independence Day, I have no idea how much it will make, but I know many who are excited for it, and that film is endlessly rewatchable too. There’s plenty of other popcorn flicks that even do well, but they’re quickly forgotten and no one claims they’re great, or they’d want to watch them again.
So, yeah, I think it just depends on what the goal of the film is. An art film, a heavy drama, maybe highly topical stuff, probably only need to be watched once, and possibly even best when they aren’t. For the rest, yeah, I think rewatchability probably helps the case for making it great.