When they do, we make an experience counter
Cursed Mirror catches any relevant ETB effects upon hitting the battlefield, and it doesn't take much to get extra value out of it
If I'm ramping, I want the most mana for my buck, so now I'm looking at Worn Powerstones and possibly even Hedron Archives, skipping the possibility of getting my Commander early in order to get more mana for bigger stuff
You could attack someone you know will kill it, but often you are looking to attack the player you stole it from, and they may prefer to not kill it, hoping they'll get it back on their turn
I feel like this would still be a compelling commander even without that text
Rishkar, Peema Renegade: Great way to have a mountain of Green mana