They're probably the most useful thing, but I'd also recommend at least knowledge of functional harmony (tonic, dominant, predominant). Personally, I like to know as much as possible, and in fact I find most of my inspiration comes from new (to me) music theory ideas. I also find it speeds up the trial-and-error process, because I know what usually works and what doesn't.
Not to say I always strictly stick to music theory, but knowledge of the rules means you can break them more effectively
The more you learn of anything generally comes with diminishing returns.
You spend a little while learning some harmony to back said scales up, maybe learning how to derive the chords from the scale then cool, your productions would probably get a little more advanced. Try it out, if you don't like using theory, then abandon it.
Don't take my word for it as I'm no expert, nor the most "musical" person on the forum, but this is my personal experience.
I've tried learning to play the piano traditionally, scales first, then some songs.
I've noticed some improvements in my skills, but I found it overly boring and dull and killed piano for me.
Then recently I decided to try a different approach, I stumbled upon a book called Play Piano in a flash, which suggested a different approach, learn how to read notes on a sheet, find a song you like on lead sheet, learn the chords to that song, play the melody with the right hand.
Maybe not the best way to learn, but I noticed that this, together with that little theory I knew, really helped me improve.
And as a side effect made me like the piano again.
If you were to make music in a box, sealed off from the rest of the world and all of its information, and could only bring one piece of musical knowledge with you to build on for the rest of your life, then maybe scales would be the thing to know.
In the real world, I think a more practical thing to know would be how to learn make basic major and minor chords (they form 2 different shapes; it is EASY). This would let you learn to play the chords for any song you like, teaching you what chords sound good together, which would basically lead you to knowing scales without having studied them to begin with. Even if you just knew how to make chords and just experimented to find which ones sound good together, it still seems like it would be more useful overall than just learning scales and then figuring out what chords are in them, etc. That's basically how I learned music. When I studied theory, years into playing, it just filled in some gaps in the knowledge I had built by playing. I think it's kind of a mistake to study theory from the beginning. Just learn to play what you like and it will gradually fall into place, piece by piece.