Sure, you have more cards to choose from if you go with more colours, but you also complicate your mana base
For myself, one pair stood above the rest, the Red-Blue (Izzet) Otters
A little while back, Magic: The Gathering decided to include only one Planeswalker in each Standard set
If the deck can handle playing a colourless land with no upside to get potential benefits while on the draw, then Gemstone Caverns can be maindeckable
[13] Tournament promo cards are given away as prizes at tournaments, making them quite rare and valuable
Balthier and Fran has greater power than cost, extra combat phases and vigilance, however vehicles are tough to pull off in 1v1, much less in multiplayer settings