Mentor of the Meek

The optional mana is the whole design. A long line of "draw a card when a creature enters" engines hand the cards over free; this one charges a per trigger, in mana you might rather spend deploying threats. Token swarms, weenie aggro, and go-wide builds generate a flood of small bodies that would otherwise just trade or chump, and this turns each one into a question you answer with leftover mana: pay when you have it, decline when you are tapped out, and the engine scales with your mana rather than fighting it. The power-2-or-less gate points the card squarely at the cheap, expendable creatures a tall midrange deck would never run, so it only rewards you for building the exact deck it wants. That self-selecting clause is the discipline that keeps a repeatable card-draw engine fair, since the creatures that trigger it are the ones least likely to win on their own. The 2/2 body is incidental, a soldier that dies to anything, but that fits the deck it lives in: the value is in the cards, not the creature. What makes it durable across eras is the per-trigger cost, which means flooding the board too fast can leave the engine idling. It asks a wide deck to keep mana open for itself, the same bargain card-advantage engines in other colors strike when they make you pay an activation rather than gifting the cards.