The test isn't to make sure they're "worthy" of getting their powers back.

Enough people who were depowered on M-day were threatening to commit suicide in order to get their powers back that the idea of the Five not being able to keep up with the world Load was an issue.

On completing pointless deaths.

The Crucible ritual basically exists to make sure that people who are willing to die to get repowered are really to do it while forcing them to put effort into it. You can't just walk in and let Nur kill you, you've got to make a genuine effort to defeat him.

This prevents mass suicide, discourages people who aren't willing to put effort into things, and counteracts the tendencies of people to look at the Five to mean that they can just throw their lives away and get a new one(There are already people claiming that they want to be killed and resurrected in a copy of Magneto's body.)

Note that Nur gives Aero every chance to stop the fight if she's changed her mind and says that there's no shame in choosing to live as a human.

Not to mention the major psychological difference between dying a dignified death on your feet after a duel and killing yourself becuase you don't want to live in the state you currently are anymore: Remember HOXPOX, as far as Xavier's concerned the fact that you can come back later doesn't in any way mitigate the fact that you died so someone killing themselves to get their powers back is, as far as Charles is concerned, still committing Suicide.

Does that make sense to you?

The Crucible is an imperfect temporary solution while they come up with a better way of repowering the Mutants.

(Honestly, I have to think that this is why they're so interested in getting Franklin Richards to come to Krakoa. Franklin at full power could wiggle his nose and give everyone their powers back.)