Actually, the minimum balance is 0.1 Algo per asset (including Algos).
So it’s 0.2 Algos when you opt in to one asset.
Note also that this amount can be recovered when the user leaves the system.
The reason for this minimum balance is to limit the size of the Algorand account table, as all nodes need to store the full Algorand account table.
But I agree that in some scenarios, this might still be too expensive.