I think this is a good idea and could reduce file sizes for compressing text. Moreover, people's active vocabulary is quite small, so even more might be possible. Just as an aside, a cliche was originally a single metal slug used to cast a repeatedly used phrase. So phrase-wise compression of English text might be useful.

However, I don't know the practical improvement obtained over the currently existing methods. This is a good project to investigate, I think. Even if text compression might not be important in the larger picture of data transmission on the web, this sounds interesting on its own merit.