The ability to both draw a card when your stuff is targeted (and before resolution, so you can draw a card to respond to it) and to keep your foes from drawing more than one is very powerful for a 4/4 3-drop that gives you ramp, removal, and countering
Trinisphere considers the total cost after applying all increases and reductions (from effects such as Thalias and Goblin Electromancers)
All duals are on the Reserved List and required for Legacy and Vintage as well as Commander
I mean skalds are like in your face warrior-poets, so I can see it
If you can tell me what cards are on The List and what their pull rates are by heart, please let me know
The concept is quite similar in Constructed formats, but following the advice of MTG player and mathematician Frank Karsten, the optimal range for adding another color to your deck in a 60-card format is approximately 12 to 13 sources of mana per color