If the spell has any mandatory additional costs, those must be paid to cast the spell
Snapcaster Mage Snapcaster Mage went from the best blue creature ever and a blue staple across multiple formats to a barely playable card
Put a stun counter on that creature.: Proliferate
This isnt a bad thing, but its worth noting that red is a ways away from having the critical mass of playable staples that blue, black, or green can boast
It obviously works well when you have multiple copies, but even on its own, a 1-mana 2/1 in an aggressive artifact-based deck is still very good
The stat boost is a decent bonus, but more often than not we'll be sending the aura right back where it came from so we have it ready for the next volley of removal