As of now, there is no purpose for Moons on r/Cryptocurrency other than paying 10x more for a monthly membership. The tipping feature is nice, but it is rarely used due to the potential loss of eligibility for the 20% bonus. Users should be allowed to tip up to 10% of moons received in the previous distribution before losing eligibility for the 20% bonus for keeping their moons in their vault.
One argument against this change is the idea that users might use this to sell their moons while retaining eligibility for the bonus on the next distribution. By capping the amount of moons withdrawn to 10% of the previous distribution, the effect will be greatly reduced. The increase in attention due to the new use case of tipping users for providing good discussion should make up for this as well.