These Gash Cafe segments are a treasure trove of characterization.
Sure... putting certain world-building info here that would make more sense being exposed in the main story is a little strange... but at the same time... the fact that this allows Haiku-sensei to make interactions between characters who have a very low chance of meeting in the story because of many factors brings possible dialogues that are truly interesting.
The fact that Day comes from what is more of a "different dimension" and her "planet" isn't actually a celestial body is a very cool detail, considering the story's progression, it wouldn't make much sense for it to be brought up unless many steps were made beforehand, but again, depending it would only swamp the pace.
Another thing too... it gave us some more info on Elleby, that explains her talents in magic; in Day's world, magic is used for "mundane" things, but in Elleby's, magic is treated more as a discipline of higher thought, practiced solely for the deepening of knowledge and understanding of reality.
Knowing Haiku-sensei's style, I bet he created this little extra exactly because he wanted to give a chance for his characters to show their personalities a little more in a situation that wouldn't make much sense to spice it up a little.