They are completely optional but I like the content of most of them. I have nearly all of them because I picked them up here and there over the course of several years. As someone else has said, the cost of going to movies, etc, has gone up and these are replayable. I picked some of them up as bundles or sales.
I really think they are just customization options. Like the theme? Go for it. Don't like the theme? Do without. I like that I don't have to repeat content a lot.
I always reviewed all of the content before making a decision. A few of them towards the end were definitely one that I just got on sale. I think they improved after the first few like Spooky and Luxury Party. I use Spooky now that I have Seasons and so I only got it around then. Luxury Party is the one I could probably do most without but I've installed the chocolate fountain at popular wedding venues.
I held out on MFPS because the theme really hasn't appealed to me and the CAS/BB wasn't strong enough. Maybe when I have a new round of kids it will make sense to add small pets, but I'm in no rush and again, it's totally optional.
I like when SPs have added things to do like bowling, movies and other hobbies. I'd see more of them get released if they add more small activities.