(similar disclaimer: I've done a bunch of text input work for mobile)

That phrase set was explicitly designed for text entry (typing) experiments. While not optimal, it does allow for more direct comparison to a large body of previous studies using the same phrase set (and similar procedures).

Having said that, keyboard input methods that provide suggestions/corrections at multi-character or word level features probably also benefit from longer words since the recognizer has more signal to work with. Revisiting the assumptions that went into that initial phrase set (character at a time input) in light of modern text input techniques might be a good thing.