Before you can evaluate if it’s convenient to communicate better with others, you need some room to explore if it helps better grab something by yourself.

Also sometime it’s nice to express something for your own private enjoyment. Maybe it can spit also to something useful for the rest of the world here and there. But making this "predicted usefulness for the common" a requirement is depriving humanity of all this intimate joys and everything that can emerge from it.

In handwritten notes I've done it many times. Mathematicians seem to create them.

On computers I haven't had the opportunity. When adopting new technology people commonly don't imagine the applications - even expert technologists, even those who developed the technology, often don't know.

> Would the added expressive power make up for the inconvenience of having to explain the meaning of the novel symbols?

It's a good question, but I could see definitions working efficiently in many situations. If the symbol is shorthand, it's easy to say 'the population of Hacker News P(H) ...', and it works well in mathematics, 'let Q be _____'. (These are imperfect examples because I can't create new symbols.)