Ask HN: What's wrong with including code in JSX?

I read comment threads where people are really against code being in JSX. But I do it all the time. I figure that if I can write an extremely terse function inline then the overall complexity of the code is decreased. I don't see much point in creating functions for one or two liners that will only be called from one place in the code. So why am I wrong? What's the inherent problem with putting code in JSX?