Why Chaldea Lunchtime?

Because you may need those sq as soon as possible for a specific banner, of maybe you have lots of servants you want to max bond, and oh do we know that takes a lot of time.
The game isn't hard enough for you to need super strong CEs, so using Chaldea lunchtime doesn't really affect your team, and gives you some extra bond points.

you can farm approximately ~84 prisms a day, this gives you over 7k prisms farmable over the duration of the CE's release, this is also while investing the gold embers into servants. there's literally no drain on resources unless you need to spend an entire month's worth of energy on farming something else or an event, where they mostly refund you 1k prisms, 100 silver embers to burn and a bunch of apples so there's really no reason not to go for these prism costing CEs