For example, a creature that enters the battlefield with one +1/+1 counter on it won't receive an additional +1/+1 counter
Historically, unfair decks abused mana better than fair decks whether that be Storm, Amulet Titan or Krark-Clan Ironworks
This is an excellent card to have in play just in case you've got three mana available at the end of your last opponent's turn so that you can bank it as energy before you untap
Songs and actions like A Whole New World refill your hand and set up combos, while disruption cards make sure your rival never feels safe
Your opponents wont be attacking or removing it from play in fear of what can come after, be it Omniscience, Cacophony Unleashed, Battle at the Helvault, or what have you
Based on the latest market prices tracked on TCG Stacked, The Masamune is worth about $4.98 USD for a regular copy and $7.15 USD for a foil copy