What mechanic best represented having access to only white, blue, and black, lacking red and green
If you really think about it, there have been cards of every type that have been too good, overpowered, and unhealthy like Skullclamp for equipment, Jace, the Mind Sculptor for planeswalkers, Yawgmoth's Will for sorceries, and so forth, but Wizards wisely realized that you don't just give up on equipment altogether because they messed up with Skullclamp or planeswalkers because they had to ban Jace, the Mind Sculptor in Standard
Often times giving your opponents a choice is less than ideal, but the worst they can do with Druid of Purification is choose nothing
If you control three or more snow permanents, your creature not only gets a +1/+0 boost but also indestructible before it throws down