Well, why not? Many people had 5 to 6 girls to get in only 4 days. You can be very lucky and get all of them without using kobans but it's very likely the opposite.
Personally, I had 5 epic girls to get. In addition, I had 1 boss girl to get from Finlamecia, 1 from Edwarda and 2 from Sylvanus. Using kobans was a good way to speed up the drops on both event and boss side. By using all the kobans I had, I have got 2 boss girls and 1 event girl (6k c.a.). With the "free fights" I have got 2 more event girls. Now I am missing 2 more event girls and I will feel very lucky if I get even one of them by the end of the event.
In general, I don't normally use kobans on day 1 unless it make sense to me using them. If the event has got many girls, I'd go with kobans on day one, otherwise I probably wouldn't. The anniversary event is an example, where I fired up 15k kobans in one go (and I have got a lot of haremmettes but not all of them). This one made sense to me as well as I missed 5 girls out of 6.
Hope this answers your curiosity :)