Originally Posted by
Shawon
Here's are two examples of this: I faced two UW Miracles last night on MWS. I resolved a Tezzeret against the first guy, made him cast two Terminus in a row, and after he bought some time with Energy Field + Rest in Peace, he E-tutored for an Helm of Obedience. He cast his Helm of Obedience with one mana leftover, and I countered with Pierce. Helm was the only way that card could win. Even if he had two leftover mana, he would have had none to activate Helm. And obviously, next turn, I assembled enough artifacts to be able to one-shot kill with Tezz. Against the other guy, he had Energy Field with no RiP. This bought him some time, except he was mana screwed at three mana the whole time. The game-ending play occurred when he cast a Rest in Peace, I Pierced it, and he FoW'd, as a result his Energy Field was sacrificed before RiP entered. I won next turn with my Cranial Plating-assisted army the next turn.
Hoarding U mana isn't as bad as you're making it out to be. If I have a surplus of mana in the midgame, then I'm not really holding myself back by keeping a land or Opal for Pierce. Sometimes I do have to make the call of not developing my board because I need to hold U open. But I have plenty of experiences of tapping out instead of holding mana for Pierce only to be SnT'd next turn or my opponent dropping something else that I didn't want to see resolve early like Energy Field.