The concern isn't really the garbage but the kind of overall eco-system it create and the ways people are encouraged to be creative in. In a platform that allow 64K character articles one might be encouraged to write long detailed posts. But 999 word limit would make people write short but creative content. There are only so many things you can say with 240 characters. But 999 words can give you best of both worlds. It's never too long. Easy to finish reading and still the content can be highly valuable.