Tmesis is interesting because it demonstrates that even in linear literature, the reader is in control of the path she reads. The author cannot control what the reader will not read. Even WilliamGibson's Agrippa that only presented each page in order, and then deleted the previously read pages, could not force the reader to absorb each individual word. Most texts don't benefit from active software controls like this, and readers are free to jump anywhere in the book. How many people read the last page of a detective novel before getting to the end? How many people go back several pages to remember an unappreciated event?
While the act of skipping text may seem trivial, or even naïve, it's integral to the act and joy of reading. It's unlikely that if someone forced himself to read War and Peace word for word, he would enjoy it. Not many people would spend three days to decode one sentence in James Joyce's Ulysses.
Tmesis is not skimming text. Skimming text is not part of the joy or novelty of experience of reading. Tmesis comes from a necessity to understand a text. That is, skipping parts by the reader's choice to move onto something new and interesting, not to find something of interest. Skimming is merely searching without contemplating the skipped parts.
Taking an example from the web, consider that the author forces the reader to pay attention to the links if the reader wants to move on from the page she is reading. Clicking on a link before finishing the current page is a new form of tmesis. However, consider when using HubAndSpoke navigation, readers often move back to previous pages and then search for unvisited links (blue instead of purple). This skimming is not tmesis.
Thus, interacting with the text is not a novelty, but an expectation. The new part of the new media is that the author can now control and structure the interaction, not just the reader. While this was always possible, it's the idea that this is something the author should do is new.
Read the excellent CybertextPerspectivesOnErgodicLiterature for more on this.