I agree that a knowledge of music theory is helpful for writing music, even for just a catchy tune. I don't know much theory myself, but I'm always up for learning because I know it'll help in the long run.
For example, if you find a good starting for your tune, and it happens to be in the key of c for example, then it's nice to know that that ending the tune with the keys c g or e can help make it sound more concluded if you so wish, and ending in d or f can help make it sound like something's gonna follow.
Plus, if you want a happy catchy tune, using a major key might help it sound more cheerful, while a minor key might help it sound more melodramatic or whatever.
Another example is playing two keys in a major key that are next to each other might be good for adding a quirky touch to the tune (sorry, like I said, I'm not too proficient at mtheory, so the name escapes my mind, but it is definitely something I picked up from studying music).
Also, studying music opens up sounds that you might have never thought of had you not taken the time to learn about music, for example when I was younger I never realized grace notes existed until I learned a piano piece that had them, which also led me to add trills to my musical arsenal.
Principles like these can be used for any form of music, and to get the impression that a formal study of music is only good in composing harmonious symphonies or classical music is ridiculous (not directed at anyone's post in particular, just saying).
As for it being limiting, it's certainly not, because, as the old saying goes "rules were made to be broken", so once you know what rules there are, it's easier to go and break them whenever you want to see what happens. It's very empowering. I think it's funny that someone implied that using accidentals was an example of "breaking" music theory "rules", when in actuality it's just using the musical principle of accidentals, and if you know what accidentals are by studying music, it will probably be easier for you to use them.