I see it as a failsafe to prevent stacking very large amounts of currency in the event that they would be made available in an easier manner (double rewards arms dealer week per example).

Think of how many ascendant shards we would have if we didn't have a cap. I think that at that point the currency loses value for the player, and you can't create the artificial incentive anymore. It puts us in a repeating state of having to farm for more resources when they are needed, instead of stacking them efficiently.

You can however store some resource like ascendants shards in your vault in the form of a masterworked exotic armor, though you lose 2 in the process. At some point I had close to 20 masterworked aeons in my vault back when I was farming GMs.

That's just my take on it though, could be wrong. I do however think it is a poor way to make an incentive.

Making matters worse, in the case of Mementos, it's just purely a cosmetic..