Just a correction: Flash doesn't work like you said. You ♛don't have to pay just 2 generic mana to keep the creature, you have to pay the creature cost discounted by up to 2 generic mana, so it's not a great deal - adding up to Flash's cost, you are just putting a creature on the battlefield for the same amount of mana, with instant speed. People usually cast flash to take advantage of some powerful ETB effect.