Artha is for more than spending.

For a con scenario it works well to preload characters with some artha so you can demonstrate the "spending artha" part of the game. However, you should still award artha at end of session, even if there's no possible way the players could spend it.

Why?

Awarding artha encourages people to reflect on the session. Every question requires you to think back on play to answer it, so wrapping up a con session by awarding artha will get people to think about the game in the way it wants to be thought about.

Artha is for more than spending.

This is what I've seen running BW and its derivatives in con slots: It helps to provide some artha, and asking the artha questions at end of session helps with wrapping up.

It works well to preload characters with some artha so you can demonstrate the "spending artha" part of the game. However, you should still award artha at end of session, even if there's no possible way the players could spend it.

Why?

Awarding artha encourages people to reflect on the session. Every question requires you to think back on play to answer it, so wrapping up a con session by awarding artha will get people to think about the game in the way it wants to be thought about.